how much do you think it'd cost for bath with provider and missionary at a Nevada brothel?

It's negotiable depending upon the ranch, the girl, and the time of day.  During slower periods, it'll cost you less (obviously).

I've only had one experience at the Nevada ranches and it was at the Chicken Ranch outside of Vegas.  It cost me $700 for nearly four hours that included time in the shower and the pool with a lovely lady.