Depends where you live and the time of the year. Clams in particular can be dirt cheap.
Oh whoops - I just remembered that once upon a time in Orlando you could get them at like half a doller an oyster. Here in the UK you can easily spend up to £5 a shuck
Not that this is a practical solution for everyone or that it's easy, but oyster larvae/seeds are remarkably cheap. 1 mm seeds are $10.00 for 1,000 and 40 mm seeds are $135.00 for 1,000 in Maryland. So $0.01 per oyster or $0.13 1/2 a piece if you can't wait long to eat them.
They are when they are the canned variant!
I've only ever checked the prices at restaurants. Maybe I should see about having these at home!