$500 is a lot to replace PS seals.
I have used various stop-leak products and, quite frankly, they do not work. Some actually make the pump leak even faster, draining the reservoir in a day. Also, the PS pump seals are so insubstantial in size that I don't see how the stop-leak additives can work on them. There simply is not that much to swell.
If you are leaking from the front seal, you will eventually have belt squeal and oily engine bay issues -- which is the point of no return where you have to reseal the pump. If you are a do it yourselfer, here's a link to how I resealed mine, with photos:
Checklist: PS Pump Re-Seal
The most labor intensive part is removing the pump from the car -- unless you know exactly where all the mounting bolts are. then it is pretty fast. Depending on which pump you have, replacing the seals takes only a few minutes.