Depends on how big you roll the joints.

Standard size ones, by that I mean ones you buy for $5, from an 1/8 of an ounce, you can roll about 6-8 joints

Get yourself a 1/4 of an ounce and you can roll about 12-18 joints, so the answer is a 1/4, or an 1/8, give or take a few joints, but a 1/4 is better for your money.

As for costs, it depends on your area as it varies greatly from city to city. Anywhere from $25 (dirt weed)-$120 (high quality) and yes, I have read here that people pay that much for weed in the US. I see the flag next to your name so I'm guessing that's where you're from...? Ask around from some fellow Americans in your area to get a better idea on cost.