I'm happy to be corrected on any of this. I've only gone with official gaming slots. You can game all night at most cons if you arrange it. I've also not counted any very late on Sunday slots because most people, well me anyway, need to get home before too late on Sundays.
So here they are in decreasing cost order:
Furnace £98 for two nights + £20 registration 2 days 5 slots £118/5 = £23.6 per game (if you stay for one night it's only £13.8 per game)
Games Expo £90 for two nights 5 gaming slots £3 each £105/5 games = £21 per game
Consternation £100 for two nights (approx) £15 registration 6 slots £115/6 games = £19.17 per game
Constitution £100 for three nights (approx) + £35 registration 8 slots £135/8 games = £16.88 per game
Continuum £90 for two nights + £20 registration 7 gaming slots £110/7 games = £15.71 per game
Consequences £75 for three nights + £20 registration* (or £3 per ticket) 9 slots (LARP) £95/9 games = £10.56 per game
Conception £80 for four nights 13 gaming slots £3 each £119/13 games = £9.15 per game
Indie Con £75 for three nights 9 gaming slots (or £4 per ticket) £75/9 games = £8.33 per game
The three least expensive cons all use the same venue.
*If you register early.