Unfortunately, almost all quality 100mw 532nm or 520nm lasers are ~$100 USD (or at least that I've seen). I have noticed that with green (mostly 532nm) lasers, you can usually get roughly 1mw per USD.
However, eBay/303/301 lasers have been known to be highly overspec (either that or they're completely dead), so if your budget can't be modified you could take a chance on a few of them. They won't be waterproof or IR filtered, but if you get lucky you
might get up to 100mw of green output (but even if you do, the duty cycle on the cheap greenies is terrible).
Generating green light is substantially more difficult and expensive than red or blue, because you either need a powerful pump diode and a crystal set (for 532nm) or an expensive diode (for 520nm). Either way, it's not cheap. :undecided:
Also, don't forget the goggles!
EDIT: Actually, IIRC O-Like has 100mw greenies for ~$50. They don't exactly have the best reputation, but they're not the worst either.