There are a lot of factors in determing a stallions fee. Conformation, Bloodlines, accomplishments, money earned, offsprings earnings, color, cuteness

or eye appeal. Then comes wether or not the stallion does live cover, AI with cooled semen, AI with frozen semen. When you start shipping semen there are a lot of other cost involved in getting the semen where it needs to go and those fees are passed on to the mare owner. Normal starting fees for a new and up coming stallion (lowest) around $300 but normal intro fees start around $500-800. As a stallion proves himself then his fees begin to go up. A good stallion that is well known and has earned money and so have his offspring can easily be $2500-3000 and it just goes up from there.
But you have to understand on the end of owning the mare, they don't always get pregnant on the first try and there is more cost that the mare owner has to worry about then the stud fees alone. An average heat cycle can cost a mare owner depending where you live anywhere from $400-1000 per cycle.
Hope this helped