Rhythmic
woog-woog sort of scraping / rubbing noise that speeds up when the car speeds up?
If so, it's probably a warped disk or bent caliper or something similar. With disk brakes, there's a smaller smooth wheel mounted on the axle just inside of the car's wheel. That's the disk. The caliper is a flat part-of-a-disk that the brake system clamps on the disk. The friction of the caliper on the disk stops the axle (and wheel) from turning. Sports cars often have huge calipers that are visible from even ten feet away, through the spokes of the wheel. Sometimes the sporty calipers are painted bright red.
The calipers are normally very close to the disks. If anything's bent (or something foreign's stuck real good on either piece), then it can rub as the wheel goes around, and hence the noise. It's
often cheaper to get this fixed if you get it fixed sooner. The rubbing can cut into the disk (potentially expensive) or wear down the brake pad on the inside of the caliper (semi-expensive). Sometimes the caliper's just bent slightly and can be straightened out. Sometimes it needs to be replaced. If the caliper is so bent that it needs to be replaced, then it may fail during hard braking, so it's a bit of a safety issue.
In any case, the noise will only get worse. Sometimes the problem is minor, though, and the mechanic will tell you that you can drive another six months without worrying about it -- and that it'll get REALLY loud when it really needs to be replaced.
But if that's what it is, and your car isn't specially hard to work on (a few imports, basically), then my guess is that it could cost anywhere from $75 to $700 worst-case. The worst case would include them looking at the brakes and saying you really ought to have work done on all of them. That could be true -- it could be that your brake is acting up because a pebble got in there, or it could be that the brakes are just worn down and ready to be replaced, in which case odds are that both sides are worn down.
Brakes are important, though. I'd get them looked at sooner rather than later. You can always take it one place for an estimate, tell them you need the car and they can't work on it right away, and go somewhere else for a second opinion. (You don't have to tell the second place you already had it looked at elsewhere.) If you actually find a garage where they tell you the car's so effed up that they can't legally let you drive it home, odds are extraordinarily good you're being scammed, but that kind of thing doesn't happen so often nowadays.
