Is this a newish wheelset on the bike? $50 to fix it is about right once you buy a cog and lockring, but it's a tad steep for just spinning on a cog suicide style.

For a little over twice that, you can buy a whole brand new wheelset (both wheels) with a flip-flop track hub on the rear. Then you can have a proper fixed setup and a freewheel. Just flip the rear wheel around to switch modes. You'll still need to invest another $30 to $40 for a cog and lockring, but this is a much better investment than throwing $50 towards a suicide hub on an old wheel (if that is the case) For cog and lockring, Dura Ace is the way to go unless you need cog bigger than 16 teeth, in which case EAI or Surly are good products that go bigger. Any wheelset under $150 that is built on Formula hubs is a great setup for the money. I'm running Formulas and Mavic CXP22s that I bought prebuilt fron Nashbar for about $140 back in January. They treat me great and have almost 2000 miles on them already. There are two or three threads bobbing around here with links to sub-$150 wheelset deals, most of which are on Formula hubs.