From experience the MPG is more than 10% less. My last LPG car did 38 on unleaded and 30-32 on LPG. That was an official Rover conversion.
Again from experience dealers don't know what to do with LPG cars, and therfore give lower trade in offers. Well they try
.
The problem here is that most cars take the coversion well and the tank is hidden discretely in the spare wheel well. But in the MINI no spare wheel well, so you lose most of your boot space. This wil seriously limit your resale options. So you should probably factor in some money for removing the kit before you sell. Not to say you won't sell with the kit.
Calculate how much you'll save and how long it will take to recoupe the conversion cost here
Savings Calculator
A search for LPG on M2.
http://www.mini2.com/forum/search.php?searchid=4895172 
