How "gas efficient" are non blowback airsoft pistols?

Well, they are pretty efficient. Since they don't use extra gas to "blow back" then it is safe to say they are probably twice as more efficient than blow backs. Propane is CHEAP, really cheap. Like $3 for a huge can and using that with a NBB you will hardly even be spending money on it. NBB are not as popular and are harder to find however. I am not sure how good the Y&P is so I can't help you on that. All I know is that is has been around for quite a while and I don't know of a brand called Y&P so it probably isn't the best brand.