Self explanatory. What is the average cost you'll pay for flyers to promote one of your bands shows? Also, how much of a necessity are flyers?
500 cost us £25 but that was a pretty decent deal...
The majority of people will just bin the flyer with out looking at it but some might come... Try and put what drinks offers there and stuff to attract people. Or say that they can get £1 off the door price if they bring the flyer...
Also check out local laws cause I know where I live you need a license to go flyring and can get a hefty fine if you get caught without one.
Depends on a few things; size, color/B&W, type of paper, and where you get them done.

I get 500 standard 8.5x11 paper, black and white copies from Kinkos for $25 (5 cents a copy), I think color copies were 8 cents a copy.
10$ for 100 pretty much. I don't get more than that too often because my band is shit at helping put them out (even if we get a start, we end up stopping before we run out :/ )

Plus, it helps to make smaller flyers and have multiples on each page, so you get more bang for your buck. I've moved onto windshield flyering instead of handing them out and posting them on walls and posts for now. It's an in-between way that helps cover many people without dealing with the awkwardness of going to public events or dealing with soliciting laws.