The mypsace.com sold for alot. I forget exactly what it was but that sold for like 35k. You just gotta hit a gold mine with a misspell that has alot of traffic.
As for replacing letters, I think most people use it as a ploy to sell a domain (the buyer thinks its corrent spelling, yet its not). If you do decided to try and sell a few, make sure to be clear its a typo!
sometimes the z is alternative spelling so is OK, but in general only works with 'non-commercial' domains and where a nice name may be secured for development.