exterior to interior outlet
It's called an inlet.  They don't need to cost $90 (unless you really want them to). They are odd, but I would still expect to find a NEMA 5-15 inlet in the $15-25 range.   

Keep in mind electrical parts don't lend themselves to online sale, because they are low in value, common as dirt, and heavy of weight.  Your best prices for anything slightly oddball will be at whichever electrical supply house doesn't mind giving you trade prices.