how big?

[zeppelin420]

How big of an amp will i need to play shows (mostly hardcore shows for people to mosh to) and how much will one of these amps cost (Canadian)

[xhaereticusx]

That doesn't really tell us anything about the size of the show. A larger combo is all you ever need though.

[zeppelin420]

how big is larger?

[TheSpokenWheel]

how big is larger?
as big as they come, the next step up is a Half stack, which is also good for shows

[Corkofski]

as loud as you need to drown out the drums?

[Dan 1616]

Depends really. I'd say about 40 watts tube or 80watts solid state for practicing and such. To be honest any decent sized venue will most likely have a PA system so your amp can be miced up and will be plenty loud enough.
As far as cost goes, you'll get what you pay for. If you're on a lowish budget you might want to look at things like:
Ashdown FA40

Marshall DSL401, avt's aren't too bad if you can't afford or don't want tube.

Line 6 spiders have some nice tones, but are quite an aquired taste.

Peavey bandits, or the new valve kings (haven't tried one so i'm not sure on that)

I think Crate make some cheap tube combos now also, which you may want to check out.

I'm not sure on the prices in Canadian, but i'm sure you can look them up.