Well, first of all, your question is very ambiguous. In addition, there is a change in person since you stated “model’ (3rd person singular) and then followed up by using the object pronoun “them.” Is it one model or more than one? Ambiguity aside, I am going to presuppose that it’s your magazine, (you are the publisher and/or owner) and that the cover image in question is owned by the magazine. Predicated upon the aforementioned, you could charge the cover model any price you deem fit. With that said, however, it is not very common to charge models for a copy of a magazine cover. On the contrary, usually free copies of the magazine are given to models or to other contributors to the magazine.
Usually when a magazine does a print run (utilizing an offset press in most cases), there will be additional copies made above the quoted print run. First, it should be said that offset presses, such as a Heidelberg, take time to warm up before they do an actual print run and, as a consequence, additional copies of the magazine are invariably made to test the color output of the equipment. If a magazine’s print run happens to be 20,000, anywhere from 100 to 300 additional copies may be printed at no extra charge. In addition to this, a magazine publisher may request that a printer run “Additional Thousands” often abbreviated as “Addl M’s” on a printing quote. Additional thousands are magazines that are printed above the quoted print run. So, if the print run is 20,000, a printer may charge $240 dollars, just as an example, for each additional 1,000 magazines above the print run. The additional magazines are often used for promotional purposes and, of course, to give to potential advertisers, models and contributors to the magazine for free.