My local Hobbytown USA stocks Wildcat 10% nitro 80/20 synth/castor fuel for $10.99 per gallon with the Patriot R/C brand name on the jug. It's awfully tough to beat that.

You could mix your own FAI fuel five gallons at a time for less. One gallon of castor oil is $19.99 from Sig, just mix it with four gallons of methanol from the local go cart/racing shop and you should be good to go for about $7 to $8 per gallon.

I wouldn't recommend that particular mix in a Saito or YS 4-stroke, but it should run most sport engines just fine.