Cost of liftable uc + extra service costs + extra insurance = less or more than cost of fuel saved?
Certainly a fixed gear is cheaper to make. Whether it is much lighter I don't know because it needs massive fibreglass cowlings, with big supporting hardware due to the substantial aerodynamic forces. Plus, it covers the wheels so the brakes cannot be visually checked and traps grass etc, so many schools take them off and take the large (~ 10%) fuel consumption hit.
Service costs are debatable too. Retractable gear does cost more but not significantly so in the great scheme of things. The biggest thing is the need for lubrication, which doesn't go well with UK's "squirt a bit of an aerosol on it" lubrication mentality prevailing in the maintenance business. If you don't lube the aileron/elevator control linkages, eventually the punter will notice and moan about it, but if the gear doesn't come down, and the emergency release fails also (because of poor lubrication) you have a £30,000 landing. My maintenance costs on the landing gear over 6 years = 0.
Extra insurance I am not sure about. I pay £2500 for the TB20 (CPL/IR, 1k hrs, 195k agreed hull value) and would challenge anybody to get below that with a Cirrus, for the same hull value cover. I think this "fixed gear is cheap to insure" was an American marketing drive, which seems to have failed even out there, and never worked in Europe. American insurance rates for Cirruses are actually eye watering.
Cost of fuel saved could OTOH be very tangible. But reducing parasitic drag is a highly desirable thing, because it gives an immediate speed gain, whereas gaining speed through power (fuel flow) is very expensive once you start pushing the 75% cruise speed values. Look at the fuel flow for the quoted TAS of say the Lancair (Cessna) 400. A bit quicker than a TB20, same kind of airframe (passenger space),
2x the fuel flow... No wonder it goes faster! A TB20 at 100% power does ~ 165kt IAS and if you stuck a turbo on it and got it to run at 100% at FL200 you would be looking at ~ 220kt TAS.