Irenicus has some good points.
I have done 1,000km from central Serbia to Greece and some in Coratia and Slovenia via ferry + a train.
I did this on little notice. I had really little idea of what I was doing. I didn't have a mobile phone. I navigated and contacted hosts via a tablet and hopping onto wi-fi networks. I was always worried I'd end up sleeping in a ditch. But all of this was what made it a worthwhile experience and helped me shed some of my green. So I think you're making a good choice.
Some notes:
1) biking 1,500 with a heavy rucksack on your back will strain your back and make every journey painful for the most way. My first journey I stopped about 200 times out of discomfort. A lot of that was down to the weight of my cira 20kg rucksack. Also having a rucksack kill increase the chance of injury when going up hill as it adds more strain on your back and legs. I biked 140km from Sofia, up Vitosha mountain. Within about 20-30km I got an injury as a result of the rucksack and biked the remaining 100+ km in pain. The solution is panniers, which will cost you about 60 EUR.
2) tires are a big issue. It may take an hour for an experienced cyclist to fix a puncture and if you don't know what you're doing you will by a puncture kit that doesn't work. You need to buy the thick patches, not the little plastic ones. However I'd recommend getting solid tires. But that's about 100 EUR and you have to get the right tires and they don't have tires for every model. But either way, you'd paying 40 EUR for good repair kit or more for a road bike.
3) lights. I feel pretty secure when I'm biking as I have a 800 lumen rear light that I have on full-flash mode in the day. You get a lot more respect with something like this. 200 EUR.
4) planning route. I would highly recommend to plan out the on back roads. I just woke up in the morning, sometimes with virtually no food and as little as one hour sleep and just headed out on the suggested route. The backroads were great. In Serbia I probably bike 50km on backroads and saw about 5 cars. But I ended up on some really busy roads in Bulgaria and Greece that were really dangerous. In Bulgaria, after scaling Vitosha, I had about 60km on a busy road with little space for cyclists. It's a really horrible experience you'll want to avoid, but if you're biking in somewhere like Bulgaria there might not be many routes you can take without huge detours.
5) cheap bikes. if you buy a cheap bike the more likely you are to have a catastrophic problem, i.e. a spoke goes. I had a bike that happened to three times, leaving me to walk home for about two hours. I also bought a bike in Serbia for about 200 EUR, bike about 200km and the cassette for the gears broke. I've had cheap bikes that didn't have problems, but some did. None of my expensive bikes had issues.
6) protein. You need high intake to avoid injury.
If you want to get there cheap you are looking at Couchsurfing, which is going to be difficult to get a place to stay at. In my experience couchsurfing is a 1 in 3 chance they will let you stay and if you are talking about smaller cities in the Balkans then there are few active members.
You don't want to be carrying food with you. If I was you I'd take as little as possible. About four changes of clothes, a bike lock, some money and a phone you can use for GPS.
If you don't have a bike I can't see this costing less than 500 EUR.
I have done 1,000km from central Serbia to Greece and some in Coratia and Slovenia via ferry + a train.
I did this on little notice. I had really little idea of what I was doing. I didn't have a mobile phone. I navigated and contacted hosts via a tablet and hopping onto wi-fi networks. I was always worried I'd end up sleeping in a ditch. But all of this was what made it a worthwhile experience and helped me shed some of my green. So I think you're making a good choice.
Some notes:
1) biking 1,500 with a heavy rucksack on your back will strain your back and make every journey painful for the most way. My first journey I stopped about 200 times out of discomfort. A lot of that was down to the weight of my cira 20kg rucksack. Also having a rucksack kill increase the chance of injury when going up hill as it adds more strain on your back and legs. I biked 140km from Sofia, up Vitosha mountain. Within about 20-30km I got an injury as a result of the rucksack and biked the remaining 100+ km in pain. The solution is panniers, which will cost you about 60 EUR.
2) tires are a big issue. It may take an hour for an experienced cyclist to fix a puncture and if you don't know what you're doing you will by a puncture kit that doesn't work. You need to buy the thick patches, not the little plastic ones. However I'd recommend getting solid tires. But that's about 100 EUR and you have to get the right tires and they don't have tires for every model. But either way, you'd paying 40 EUR for good repair kit or more for a road bike.
3) lights. I feel pretty secure when I'm biking as I have a 800 lumen rear light that I have on full-flash mode in the day. You get a lot more respect with something like this. 200 EUR.
4) planning route. I would highly recommend to plan out the on back roads. I just woke up in the morning, sometimes with virtually no food and as little as one hour sleep and just headed out on the suggested route. The backroads were great. In Serbia I probably bike 50km on backroads and saw about 5 cars. But I ended up on some really busy roads in Bulgaria and Greece that were really dangerous. In Bulgaria, after scaling Vitosha, I had about 60km on a busy road with little space for cyclists. It's a really horrible experience you'll want to avoid, but if you're biking in somewhere like Bulgaria there might not be many routes you can take without huge detours.
5) cheap bikes. if you buy a cheap bike the more likely you are to have a catastrophic problem, i.e. a spoke goes. I had a bike that happened to three times, leaving me to walk home for about two hours. I also bought a bike in Serbia for about 200 EUR, bike about 200km and the cassette for the gears broke. I've had cheap bikes that didn't have problems, but some did. None of my expensive bikes had issues.
6) protein. You need high intake to avoid injury.
If you want to get there cheap you are looking at Couchsurfing, which is going to be difficult to get a place to stay at. In my experience couchsurfing is a 1 in 3 chance they will let you stay and if you are talking about smaller cities in the Balkans then there are few active members.
You don't want to be carrying food with you. If I was you I'd take as little as possible. About four changes of clothes, a bike lock, some money and a phone you can use for GPS.
If you don't have a bike I can't see this costing less than 500 EUR.