Escaping to mainland Europe on a budget

For that budget, look further east, you are going to be constrained by the cost of flight so fly wherever is cheapest. But look at flights to the Baltic States, Poland, Czech Republic, Slovakia, Bulgaria, Slovenia and Croatia. If you can lay your hands on a tent, so much the better. Keep away from the big cities (trains and busses are cheap in these places).

Flights midweek and at odd hours are cheaper.

Have fun.

Also I don't know where you live but don't discount train/coach travel to places. London to Amsterdam is £50 return by train and boat, less if you are young like yourself.

I live in Shrewsbury (Birmingham is the nearest airport).

have a look into eurolines coaches

Eastern Europe sounds really appealing. Anywhere you would recommend?

That's where I fall down, I've been to St Petersberg, Moscow and Berlin but nothing in Between. I've heard good things about Kracow.

Budapest! I've never been there but my friend is currently there and says it's wonderful. Apparently it's the new Prague(with less tourists of course).

I'm with Ed on going further east although I'd recommend you go to some of the big cities, cos you'll miss out on some wonderful experiences.
It's probably a little late to apply for this, but four years ago I went to Poland for a month to teach at this summer camp. You don't need TEFAL - it's more about conversation and culture. I had to pay for my transport (coach and boat from Victoria Greenline about £80) and spending money and that was it. We didn't get paid, but we did get around £3-40 in pocket money and a free coach tour of Poland at the end, with accom and meals. We were near the Baltic coast, so we could even go swimming.
The Polish kids were great (I have fond memories of smoking shit weed and listening to Illmatic with them) even if the women running the place were a bit strict. First week was a drunken haze and we all got in trouble for partying past midnight (jeez!). Anyway, that's maybe a possiblity for next year. Google APASS.
Immediately, I'd recommend Prague which is still cheap if you avoid tourist destinations. There are student halls you can stay in behind the Petrin hill and next to the derelict, vast commie era statium. They're basic, but clean, and you're only going to be there to sleep anyway. Top tip is to buy the Prague Post, the English language paper which has up to date listings and a price guide. Get trains (insanely cheap) out of Prague and visit places like Kutna Hora (crazy church filled with bones, silver mine and nice old town to explore) and numerous pretty towns with interesting buildings and galleries. Going up to the Polish border is great too, but you'd need local knowledge or Czech friends to get around the hills.
Krakow is glorious and pretty cheap if you avoid the tourist haunts. Plenty of great cellar bars. Dunno if it's still there, but we found a great boho bar/courtyard just off the main square. Followed our ears (could heard James Brown) and ended up having a great night cos we heard fellow Scots and Irish voices and got chatting to them. They were on a art school exchange and took us out to a cheap club where we danced with hott Polish girls.
Less popular with British tourists is the Baltic coast, but Gdansk is a stunning city. You can get cheap accom in student halls, the trams run until 2am, and there's plenty to do during the day in terms of historical attractions, museums and shops. Visit its twin towns Sopt and Gdynia if you want to party. That coach I got from VGL stopped in Stetzin and ended up in Gdansk. It's not so bad when you've got pals. Just try and persuade the polish guide to put on the odd English language video.
Anyway, good luck and let us know how you get on.

Hmmm, can't find anything cheap for Eastern Europe... but Eurolines do a really good Amsterdam deal for only £32 return.

I'll let you know what happens.....