I have zero experience in such matters so keep that first in your mind. I don't perceive
Quote: but have never been asked for pricing where they want to reproduce my photos to sell.
this type of reproduction as selling your prints to others. At least not like artwork sales. The images compliment the product being sold; they are not the product. In my mind I would be wanting more money for art work or poster sales than I would for something like this.
I really like what thedollarbill said as a starting point. Having known and dealt with a few accountants in my time I would never sell anything for a % of the profit. Percent of gross profit after cost of goods maybe if it was really well defined what went into the cost of goods amount. It is a far cleaner transaction for all parties to negotiate a fee and perhaps a royalty per unit than any other method.
Then one should consider what if any photo credit, the exposure derived from that, what if any opportunity there would be to sell the prints as artwork or posters and how much you would like to do it. The pros and semi-pros on this forum are going to encourage you to keep the price high because that is what they would do. The truth is you need to cut a deal that makes you feel good and strive to please no one else.