H.W.
I think you have to calculate the total cost of the trip. If it comes in significantly under $500, I'd do it, personally. That could be a lot of money and if you feel it's more convenient, that has value in itself.
I had my 20 week ultrasound done at a doctors office recommended from my midwives. Equipment is no substitute for skill, and he was good. If you felt satisfied with the care you received the last times you went to the previous OB for the last two pregnancies, I'd say go for it.