Ben's Bike is selling D-A cogs for $18, that's not a bad price. I wouldn't buy any of the cheaper ones.

There is a relatively small number of quality track cogs: Dura-Ace, SOMA, EAI, the new Surly, Campagnolo, Sun Tour, Miche (maybe), Phil Wood... did I leave any off?

Anything else is liable to be a cheap stamped cog with poor materials and sloppy tolerances. There are too many people on this forum who can attest to how a bad cog stripped their hub. A decent cog lasts a really long time. Skip a beer here or there over the next year and you'll have it paid off and have many more years of reliable service ahead of you.

Or maybe you'd rather buy a new hub...