Based on what I have seen on another forum, I would say that the magazine is quite large, has a worldwide circulation in the 100K's (though mainly US), and is published by a large company that rhymes with smelter... (depending on your accent)
Am I even close?
Anyhow, what they should pay you, (And they very definitely should pay you, don't accept the "but you get full credit" crap. If you give it to them for free, another professional goes hungry and a fairy dies, or something like that
...) basically depends upon the planned use of the image, (cover vs inside), size it is used at etc etc as already mentioned by others.
They should have a published/readily available guide to their payment rates for photography, but I have not been able to find it on the website(s) of either the magazine itself or the custom publisher I believe is involved. Ask them for it, and ask them for what they say they are willing to pay B) .
If you have no joy with that, there are services out there that will guide you in the direction you should be going, based on the answers to the questions above. But there is no point asking for $1500 for a cover from a magazine that openly states that they pay, say, $50.00. As they have not been all that transparent (AFAIK), then I think that leaves you in a position to negotiate.
Oh, BTW: it is standard practice to ask MORE for an image where no credit is given, as they are denying you recognition and the potential sales leads that arise from the credit...
You will sell them the rights to use the image ONCE, in a defined market, and for a defined purpose only. If they later decide to include it in the "Best of..." book, they need to pay for that use as well.
Oh yes, congratulations. It must be a real thrill. But don't let the thrill lead you to making a potentially expensive (in terms of lost opportunity) mistake. As the other craig says, everyone else in the chain is going to profit, and so should you.