Pick me up is a great art fair/festival to spot the coolest graphic art. You can buy prints at any price. I went there last year and bought two prints from Print Club London for £5 each. Of course for very limited edition prints you'll pay (a lot) more (mine were not so limited, haha). But however, it's worth the visit and I dare you to leave empty-handed!
Soma Gallery is a real recommendation by the way. In my opinion, they represent the best (graphic) artists. For example, Gemma Correll is one of my favourites.
[Pick me up is at the Somerset House until May 5]