The short answer, Mark, is: it depends.
I’ve had professional, internationally traveled, widely-published models collaborate with me in return solely for prints and I’ve had others charge more than $150 per hour.
Generally, the range — and it varies by geographic region and experience — is between $25 to $100 per hour. Most paid models require at least a 2 hour commitment and some shoots have lasted up to 8 hours.
A number of local photographers offer time-for-lodging, hosting models in exchange for photoshoots, but I’ve got children still in diapers, so that isn’t an option.
For the moment, since prints aren’t selling and no one is hiring me for shoots, I can’t afford to pay models… which means I’m having to turn away quite a few notable travel models who want to work with me, but for whom shooting for trade isn’t an option, given the realities of living photoshoot-to-photoshoot.