I would imagine there is not a set rate that one can quote you for since different producers will offer you different sums of money based on what has worked for them and their 'models' in the past. How much experience and work the 'model' has may factor into this as well, as she may be in a position to ask for more. Unless they're a company, it may very well be that they're hiring and paying out different sums of money on a case by case basis and considering what the 'model' is willing to do and how much they're willing to pay her to do it. Because alot of these guys are amateurs or work out of their house (or simply approach people on the street), and they pay them on the spot or immediately afterwards, its hard to say what is considered a base payment or what is considered standard.
Everyone has their own ideals of what is enough, too much, not enough, or fair. If you work for a company, they may be less inclined to pay what you want because you're seeking employment from them, not them scouting you (in which case you'd probably have a little more say in what you get paid unless they're very clear they have a budget and cannot exceed it).
Different situations may mean different types of income. If its just a short video they're making of them going out and getting girls to show them their feet, who knows how that goes down.
If they want you to be nude and/or engage in homosexual/sexual activities, they most definitely should be paying you a greater sum than they would for the average activities of just tickling, fondling, etc. This should go without saying. You should be making more under these circumstances, since you're willing to go farther and are fulfilling a larger set of themes they and their customers wish to have.
If its a company you're pursuing, you'd definitely have to contact them personally and talk to them about rates and earnings. In fact, I think thats the only way you can find out, since, from what I gather, they don't release any of their financial information to the public except for the cost of their videos and merchandize. How much their 'models' make is confidential.
They always request that you inquire about it, so thats what you should do. I strongly suggest to you that if you really are going to do this to make sure they're not cheating you by giving you less money than you would be making. Scout other companies and ask them about thier rates and compare them to the ones you want to work for. Make sure you know what you're getting into and make sure you're well informed of what you could be earning.
