Sounds like I will keep an eye out for a used housing. For some reason, inexpensive used housings for the D7100 seem few and far between. I have only briefly looked a couple of times though. A new housing Aquatica housing for either the D7100 or D850 is ~$3000. So unless I can find a used housing for less then 1/2 the price it does not make sense.
If I was not planning on a D850 for my other uses, I would consider a different body.