First find out what is the retail price he intends selling the pen for. If he is good at his trade, he should have an idea of what people are ready to spend on your pens.
Don't ask him straight out what price you think we should sell this pen for: instead say something like:" I usually sell this pen for (highball price here). Do you think that will work in this store?" he might agree with the high retail price (btw, us penworkers, being so critical about ourselves most often undersell our pens anyways) or might tell you that in his area, he customers, will not spend mOre than so and so.
Once you know the retail price, expect to sell it to him for anywhere between 40 to 50%.
That's where you can handle: perhaps start asking for a high %, and than go down in price, but ask in return for things like good placement, or so.