Once as expensive is a number, it's 1 x the cost. Twice as expensive is 2x the cost. Once as cheap might arguably be 1x the cost, but twice as cheap is undefined.
It seems like we're taking at cross purposes. I'm not trying to convince you that anyone says "once as X". I'm just saying "twice as cheap" is nonsensical. Because it's a multiple of something undefined.
Nobody says that. If someone said "once as expensive" I'd interpret "once" as talking about time.
"Once" can be a counter but not a multiple.