Depends on the typo. Some are worth hundreds of dollars in monthly income. Some are worth nothing.
It depends on:
-market
-amount of traffic
-legal issues
Good markets would be ones that you can get a good PPC rate from (http://www.overture.com - search a term and then click find max advertiser bids) or have lucrative affiliate programs.
Amount of traffic... more=better.
Legal Issues are domains that *may* infringe on trademarks of companies. This can cause some problems with the value of it.
Typo domains, in my opinion, are only effective when
a. you switch a letter or
b. substitute a letter with a surrounding character or
c. minus a letter
I make typos all the time following those rules above. Like www.gppgle.com or www.hotmsil.com. You just need to study your own typos and see what people are more capable of messing up on. However don't load the typo domain with one of those annoying search engine pages, I never use those and click for the right site immediately. If anything you should add cost-per-impression ads. Hope that helps.