I always quote time and materials with a "based on what you have shown me I think it will be X amount of time", I never say Y amount of dollars, just X time. I then tell them that any change or any lack of clarity will result in the number going up or them deciding to remove the feature to stay in X. Keeps you out of the "we agreed to the project for Y amount and I did not get the project and he got Y" disagreements.
Sounds like a great idea! When you mention "time" are you referring to billable time or elapsed time? Also do you discuss a specific rate or a range of rates? I've tried your approach and ended up with clients doing a time x rate calculation and then ended up back in the Y amount debate.
