I am trying to calculate the page count of our first book. Its about 36,000 words right now, will probably end somewhere between 40,000-50,000. I know for novels the average page is something like 250 words. But for an RPG, with art, indexes and front matter, what can I expect my final page count to be? And what is the ideal range of words per page? This is important because I need to give our seller a rough pretty good estimate of the number of pages four months before release.

RPG books are a pretty artistic endeavour, so the amount of text per page is quite different for different books as well. You can't really get even a rough estimate without first planning out your layout. Once you have a rough layout plan, you can put some text on those pages and see how many words go on a page.

If you're using Indesign for your layout, this is what you can do. Make a few sample pages.  Go google some fantasy art that's approximately the same as what you'll be putting in there yourself, make some text boxes and use the option "Fill with placeholder text."  Make the sample pages so that your 4 sample pages roughly represent what your entire book will be filled with.  One of your pages will probably be all text, one will probably have some sort of table, one will have a picture...  Then use the word count, find the average amount of words, and that'll tell you how many pages you should expect to have. 

Your only other option is to finish your book 4 months prior to release.  Which may not be a bad idea, truthfully.  You can spend this time polishing up your website, showing it off to gaming communities and working on supplementary material.