As stated above just keep an eye out in the for sale secton.I got mine with low low miles(seller said 10,000mile)for $450.I have had them on my car now for 12,000 miles and they are great.You can find some with more miles for cheaper.
Might wanna talk to Stu Hagen and see who has helped him with his stock twins. Chances are, they are very familiar with the CT12's by now, and may know just what to do to rebuild them for a little more power.
When mine went out, I wasnt about to replace the stockers. IF you have ever taken out the stock turbos before, then you know it is a pain in the ass. Single turbo conversion can be done much cheaper than it used to be. It was worth it to me just to have the freed up space and the ease of working on the single. Even with a small single you can run stock fuel and keep bpu like power.