I want make a photo project to send to a stock photography agency, but I don't know what I should pay to a model for signing the model release.
Pay by photo, by hour, or in some other way?
It sounds like you haven't worked much with models. If not, it'll be an interesting journey. It's convenient to make a broad categorization of models into two groups: 1) Professional models, represented by an agency; and 2) People who model but are unrepresented, part-time, etc. If you want to be reasonably sure your model will be on location when he or she is supposed to and that you are getting what you pay for, agency models are for you. If you are working on spec, as with stock photography, you may be willing to trade the reliability of a model with a pedigree for a less expensive one.
To start with, you need a clear description of what you plan to do, where, for how long, whether the model is providing wardrobe, makeup, styling or if you will have hair, stylist, etc. on set. Be sure to mention up front whether there will be any unusual requests of the model because if your model is uncomfortable or feels you sprung something on him or her, the shoot will not go as well as if they are prepared.
In the first case, you will negotiate rates with the agency and it will be quite cut and dried. The latter case (and this is where modelmayhem.com comes in) gives you quite a range of talent and professionalism. The models range from, to be blunt, flaky to very responsible. They can be extremely talented or totally clueless. With these models, it's obviously important to have your description, but you'll want to meet them first and take a few test shots, even with your cell phone, just to see if you can communicate and work with them. If you decide the answer is yes, then you determine a rate.
There is no standard rate and stock photography is very speculative. So is modeling. Many of the Model Mayhem models are professional and serious about making a living modeling. Others, not so much. The pros will have either hourly or day rates and won't be afraid to quote them (they should be able to do this based on your description). Novices may be happy to let you set their rate. Many will work in exchange for some of the outtake images.
When you evaluate this, please bear in mind the total projected value of the shoot to you and measure your modeling fee against how much you expect to earn. If modeling and other expenses (equipment, transportation, meals, etc.) exceed what you expect to make from the shoot, just don't do it.
I know I've strayed quite a ways from the original question. I hope this is useful.
It depends on the model. It might even be free if the model is fairly new and just wants her/his portfolio in exchange for her/his time. modelmayhem.com is a good place to start your search.