Another thing to take into consideration, it is NEVER just the stud fee. There are usually booking/chute fees (sometimes included in the stud fee), vet fees, mare care (boarding) if you take the mare to the site of the stud. Also if you AI, there are shipping fees. If you wait until your mare is 10-11years old, she may have to go through a course of Regimate to get her cycle in check. Trust me, it's NOT cheap. (this also requires vet care, so more $$$) Not to mention the extras that the mare will need during pregnancy. They eat ALOT more. Plus, depending on the care you plan to give your preg. Mare, there are also supplements for preg mares that add to their nutrition during pregnancy, which are not cheap. We're awaiting our 4th baby coming next May. It is fun, however it is also expensive if done with the care that they need.
You need to really take in to consideration WHY you want to breed. Is it just because you have a mare? Will you keep the baby? Most $500 studs are average to below average quality, or are unproven. However, due to the economy, there are some nice stallions that have reduced fees. Good luck! :)