You can get Dura Ace cogs on eBay for $24.99 shipped. If you're looking for something 13-16t, these would be a good choice.

I don't like the cheap Formula stamped cogs. They wear quickly with fairly non-aggressive riding.

I do like Eighth Inch ($24.50 shipped on eBay/a lot of other companies rebrand the same cogs) and Origin8 cogs (shown in above post).