Cheap is relative. Out of curiosity, I checked the US inflation rate. For other countries, inflation rate has varied, but overall this is representative. In 1965, I believe the list price of a Cox .049 Babe Bee was $3.95 US. Rounding to $4, and compounding annually is the average annual rate found on
Cost in 2013 would be $29.16 US. Cox International's price for their cheapest tank engine, the Cox .049 Skymaster found on
is $44.95 US or roughly $45. Price of engines are more now than they were when Leroy Cox was making them. If one downscopes to a tankless engine such as the die cast Sure Start, the cost is $26.95 us or roughly $27, which is slightly cheaper.
Then there is shipping of about $7. Personally I would like to get at least 100 flights or more out of a Cox before having to replace or repair it. That equates roughly to 6 to 8 hours flight time total or a year or two of flying.
So, it depends upon one's philosophy. I'd prefer to keep flying reliably, and not have to worry about a shortened life engine. Thus, I will ensure a reasonable oil content overall with a goodly percentage of Castor oil. It doesn't have to be 50-50, but would like a substantial enough amount to ensure the engine fulfills its normal life expectancy.
As usual, as they say, YMMV (your mileage may vary).