If $40 is the cheapest blasting job you can find, and a reconditioned drum is only $20 more, it might be worth it to go for the reconditioned drum. You know what your labor is worth to you, so...
I'm with BobBill, in that you can only go by what they tell you and what you see. If it looks like it's bare metal inside (or maybe a light oil) then it might just be bare metal. I would also think they'd use a rust inhibitor, but who knows? You could hit a spot inside it with a piece of sandpaper and give it a day or two to see if any rust forms on that spot only. That would tell you if there's any kind of rust inhibitor on it or not. Of course, if the inside of the drum has a light coat of rust on it, then you know they didn't coat it.
I used a weed burner on the outside of mine to burn off the paint and any crud inside it. The burner got the metal so hot from the outside, that I didn't need to try to heat it from the inside. The main reason I had it blasted after I burned it was to clean up after the weed burner, and to make sure I got the inside of the drum as clean of contaminates as possible. I too hit it with high-temp primer and paint after that.
OTS - UDS - Don't have a UDS? Well why not?