It sounds like an excellent idea.
The maintenance cost of the runway itself is not that great. If you look at ex-WW2 hard runways which have not had any maintenance in 65 years, they are now breaking up but most of it is still there, and even the worst one I've ever seen (say Andreas on the IOM) could be turned into a brand new tarmaced surface for about £200k. The Colt runway would be good for a few decades, IMHO.
Obviously the maintenance cost on buildings is pretty significant. The solution would be to do what every other successful GA airport has done: have a load of commercial property (a trading estate, basically) on the site.