kimanand,
All of the cost plus contracts I have seen are cost plus a certain percentage rather than a flat fee as you have alluded to. They are usually used on commercial projects, and even then, mostly on projects that have not got sufficient documentation in place before construction commences for a fixed price contract, for example; lots of PC sums for items such as finishes, fitouts, demolition (possible contamination etc)
IMHO, they benefit the builder more than they benefit the client as the builder is not held financially liable for any mistakes or ommissions they make.
If you are serious about going down the cost plus path, I would recommend to cover your ar$e and get a quantity surveyor to draw up a bill of quantities so you know where you stand, and are able to track your expenses.
Boods