My personal rule is, I pay £1 per estimated hour of enjoyment. I almost NEVER buy new games full price, and will bargain hunt, or just wait 6 months/a year until they're around £10. Exceptions to this are Prof Layton games (Xmas treat! ^___^) and maybe soon Ace Attourney - though so far £5 and £8 are bargains.
However I do tend to break this rule for PC Engine games.
Anyway, I have a huge list that seems to keep growing, and if I sniff out a bargain, then I can cross it off. But to be honest, if I won the lottery tomorrow, it would kill half the enjoyment of collecting for me.