ORIGINAL: RCKen
That all depends on how much you fly. One gallon is 128 oz. Figure 8 oz per flight will give you 16 flights. Now if you fly 2 flights per day that will give you 8 days worth of flying. Glow fuel costs vary depending on where you buy it. $12-$20 per gallon is pretty average. So with that you can get an idea of how much it's going to cost.
Now I've had days where I have got through more than a gallon of fuel a day. I fly 15% Cool Power and I usually pay ~$18/gallon. So the costs can add up quickly. This is one of the many reasons why gas powered engines are so popular. Glow fuel at $18/gallon compared to gasoline at $3/gallon. No brainer there!!! This is why I'm starting to get more gas powered planes. And with smaller gas engines not being available it's becoming a better option all the time.
I hope this helps give you an idea of what it's going to cost you.
Ken
It also depends on the amount of nitro one burns and the type of performance one needs out of the power plant. For example, YS170 4 stroke used in Pattern and Scale really comes alive on 30% heli blend which costs around 30$/gallon. The YS170 uses about 20 ozs of fuel per 10 minute flight, so that gallon lasts 6 flights more or less.
I use an SAP180 2 stroke gasoline engine on pipe in my pattern plane rather than a YS170 4 stroke. The same 10 minute flight only uses about 6 ozs of gasoline and oil or around 21 flights at 10 minutes each per gallon. A 5 gallon container of gasoline and oil costs about $22 (14$ for gas and 8$ for oil). The 5 gallons of gas mix will last about 105, 10 minute flights, which is a pretty good amount of flying
Doing the math, the YS will runabout 5-6 hours on 5 gallons of 30% and cost around 150$ (exclude overhauls for the moment). The SAP180 will run about 18 hours on the 5 gallons of gasoline/oil mix and cost $22.
Convert everything to a common length of operation time of lets say, 18 hours, and you will see that the YS will cost around 450$ for 18 hours and the SAP will cost around 22$, or about 1/20. Of course this is the most extreme case and most people don't run a YS170.
Powerwise, the YS holds a slight edge at about 4 1/4 HP versus 4 HP for the piped SAP.
Hope that helps put it in perspective. Lower cost glow fuel will cost proportionately less to operate than the YS but will range from 8X to 20X what gasoline operation in this size range will cost. If you fly 300 flights per year, cost difference will be $500-$1000 annualy. That's not peanuts!!