Generally will pay more for a similarly equipped boat powered by a diesel engine than for either of the other two alternatives - and for a reason: It's the most desirable.
Gas inboard was used on many early model Cat27's and some people swear by them. Easy to work on, simple to replace (with a diesel....), but many people will say that an inboard gas engine on a sailboat is just looking for an explosion to happen.
A 4 stroke outboard, on a substantial/appropriate bracket (or a two stroke that fits in the early C27 center "well" area) can be a very good - and probably less expensive option for a person looking for a C27. Two of the big advantages to an outboard on a bracket is that the entire prop and engine shaft can be raised completely out of the water when sailing - reducing drag! An outboard can also be (relatively) easily removed and transported for servicing - try that with any inboard engine. The downsides of an outboard on a C27 can be that the engine may be subject to "pooping" (getting dunked in really sloppy seas) and that an outboard typically doesn't provide very much thrust in reverse on that size vessel.
I had a 1986 C27 with an outboard on a bracket for a number of years and enjoyed it very much. Won a number of races with that boat.
Good Luck!
Tim Brogan
April IV C350 #68 (Was Inamorata C27 #6111)
Seattle