OK Chilli, Here's the deal.
You go and buy an aircraft, let's say you drive a hard bargain and are satisfied that it's worth the 30k you paid for it and that it's market value is therefore 30k.
Now put 10k of your own money into an engine fund. I'll buy a half share of the aircraft and engine fund for half the market value, 15k. Fair dos?
Alternatively if you reckon that 5k should be knocked off if it doesn't have an engine fund I'll buy a half share without the engine fund for half what you paid less 5k. Can't say fairer than that can I?