I certainly would have interest in some products to help support Larian in developing future games. Here are some of the things that I would definatelty purchase and what I consider a fair price in US dollars:
Coffee Mug: $10 - $15 (maybe with the picture of Samuel standing outside his Citadel on the mug)
A Jewel Case: $10 (that will hold Divine Divinity, Beyond Divinity, & Divinity 2 CDs. And the Larian Games Logo on the case)
A Book of DD and BD area maps: $10 - $15 ($20 if it includes a signed picture of the development team)
A chair that does not make my bum sore after 12 hours of playing: (price negotiable)
I think these items would be rather cheap for Larian to have made (except the chair), and could bring them a few extra coins to keep the great RPGs coming.