looking to find cheap transport from Sofia to Budapest, but my internet's being no help and tells me that it'll be 136 euros? Any suggestions? What's the best way to travel in that area?

Well, you're very unlikely to save any money booking over the internet. Where are you getting that price?

In general, I'd go for train, and get a couchette for a trip like that. But the bus is probably a bit cheaper. I had some bad experiences with buses in Bulgaria (tended to have non-working A/C in buses with sealed windows...not pleasant), so it would definitely be my last choice.

It's an 18 hour trip, so I highly recommend spending a bit extra for comfort if you can. The schedule on Deutsch Bahn shows a direct train daily at 12:40p, but it can't give prices. This page quotes the price as 81 lev (€40.50) one way with a 21 lev (€10.50) surcharge for a couchette. 1st class is 50% more. I'm not sure how up to date that is, though it is the agency that sells international tickets in Bulgaria, so I would hope it would be accurate.

hmmm wizz air might be of help for you. or pretty much most of the low-cost airlines which operate in the area.

Don't do the bus, it will be hot and the border crossing could be a nightmare, unless the bus goes through Romania...?

I'd say do a budget airline if you can, if not go with the train

as far as i know the busses have air conditioning. the trains don't.

all trains going through Romania have AC, if they are international trains. I've been by train from Arad to Bucharest on an international train coming from Budapest and it was actually cold during the night (in JULY!)

If there isn't any direct train from Sofia to Budapest you can take one from Sofia to Bucharest and then from Bucharest to Budapest (there is a direct train!)