He may have figured that you wouldn't know the difference, and rim shoes are more expensive. If he gets the same amount to shoe, no matter which shoes he uses, then he makes more money off the regular shoes. Just a guess.
I'd pay more though... I never know what he's gonna charge me, it's different every time. We think he just gives us a number based on what he needs at the moment.
It's just a puzzle to me.
That is why I always thought about buying my own shoes so they would be handy when and if you can get a farrier out!