If you could could get 3 'Es' for a tenner (off a mate) or one for £15 from the shop (asking you for i.d limiting the quantity you can buy, etc), I like you have no idea what folk would go for, but i'd hazzard a guess that most folk would plump for the 3 for a tenner and no questions asked deal.
Given the ridiculously poor quality of pills over the last decade I'd go for the £15 over the counter one every time! Legalising ecstasy is a no-brainer if you ask me. It's pretty much the safest recreational drug out there, and has the added potential to change society in ways which would be highly beneficial. The only things standing in it's way are the breweries and a govt who are still stuck in a mindset that people shouldn't be having too much fun.