How to price a model

If you are experienced enough with modeling/texturing you will be able to estimate how much time each model will take you.
From there, charge by the hourly rate you are happy to work for and you will have a number.
Just don't undervalue your time and take into account all of the taxes and expenses you will have during that time.
Otherwise you will be just wasting your time.

It's difficult to advise someone on how they should price their work, because everyone values their skills and time differently, but allow me to quote a great article from Career Foundry called Pricing 101:

This is a story of two designers. Both designers have about the same skills, and both are making about $6,000 a month. One designer works 60+ hours a week while juggling a ton of clients. The other designer works about 20-25hrs per week while dealing with just a few clients. Both designers' work is very similar but one works significantly less than the other. How does this happen? One designer knows how to price his work and value his time to earn more income in less time. The key to this is pricing your work based on your skill level and how much you value your own time. The more you price your work based on a true evaluation of your own skills and time, then you will start to attract a higher quality of clients with bigger budgets.

There is software out there that you can install which will literally track your computer activity as you work on a project and give you a reasonable price based on an hourly rate you choose. This could be a good way of doing things at first because sometimes it's easy to lose track of time or forget how long you've been working on a project. Eventually, you may want to get away from charging by the hour, because clients typically don't care if you've worked on something for 20 hours or 20 minutes. They care about quality.