Yes, it sounds reasonable. Platinum is very expensive, and prices fluctuate greatly, too. $500 dlls. for a platinum 6 prong setting seems quite OK to pay to me. But verify that it is PLATINUM stamped, since some jewelry stores use little "tricks" to make you feel like you are buying platinum and instead they are giving you coated gold.

I always recommend ONLY 950 Platinum. Never buy less than 950 Platinum.
You do not want more impurities than you need.
950 Platinum typically consists of 95% Pure Platinum and 5% Iridium.
Unlike 18K Gold, or 750, too many impurities cause the metal to oxidize. Therefore losing its luster and shine very quickly- needing constant polishing.
As far as testing the ring, you need to find a local "refiner" company (look it up under jewelry-refiner in yellow pages). They can test the purity of gold and platinum- ususally at no charge.
Any store that uses Tricks e.g. stamping Platinum on a 18K white gold ring need to be reported to the attorney general.
Basically, weight separates platinum from gold- platinum is much heavier.

As far as price, you need to determine its width (3mm, 4mm, etc..) as well as finger size to get an accurate price.
Tiffany styles are typically 2 to 2.5mm in width and are knife-edged.
For example, 14K white gold top and 14K yellow gold Tiffany ring goes for around $65.