If you changed the spark plug, and tuned the engine to run on Glow fuel, it would use something like 1.0-1.5 oz per min. Lets do some math....

Glow Fuel = $20 Gal
Gas w/oil = $3.50 Gal

60 min of flight time @ 1½ oz per min with glow & ½ oz per min with gas.

60 min using glow fuel is 90 oz @ 15.6¢ per oz = $14.04
60 min using gas/oil is 30 oz @ 2.7¢ per oz = .81¢

So..... 4 - 15 min flights would cost you $13.23 more than using gas and the performance would be about the same.......

What was the question again???