Hey Bruno,
I was never quite sure if I was doing it right, but for years I did it this way and never got into trouble.
Anything that I purchased for the gallery, ie. prints, framed pieces, gift items, I would record in my expenses data base as "gallery expenses". Anything that I purchased for framing, ie. moulding, mats, glass, etc., I would record as "framing expenses". When I accessed the data base I could easily seperate the two. Sales were always recorded in a seperate data base the same way so I could see how expenses related to sales in each catagory. The problem came in when I would use scrap material (already paid for) to frame a piece for the gallery. Since I already paid for them, I never seperated the costs for materials already in the shop. Both gallery and framing expenses were combined for income tax under "cost of providing goods sold".
Inventory is another matter. I would record items in the gallery at their retail rate and place a estimate value on scrap material in the shop (usually very much below actual value)...I mean how do you do it acurately. Since my fiscal year is the calender year, I would do my best to keep my inventory as low as possible by December 31. It usually wasn't a problem because of the Christmas season.
I don't use a specific framing computer program for my business, so I don't know if there are programs out there that can help. I also believe that we can over complicate these things.
Good luck...hope this is of some help.
Ted