Good question. I just sold an 1830s mixed use building that sounds very similar to this. It attracted interest from a variety of folks from many walks of life, with wildly differing opinions as to value. This can be disconcerting unless you have confidence in your pricing. Here's a couple of ways to look at it that might help you, none of which is intended to substitute for proper professional advice from a qualified commercial appraiser, which would be money well spent.
First, ask a Commercial Broker for a price opinion using sales data. Since there aren't any recent comps in the immediate vicinity, you cast the net wider, then make adjustments for the variables. The key factors you would be looking to match are historic/landmark status, building/lot size, zoning, and foot traffic, then make adjustments for location.
Second, interested investors may evaluate the property based purely on how much income it makes, without even walking through the door. These folks are using the income approach to value and want to know what size of bank loan they can secure using the current income derived from rents, as well how much profit the building makes per year after paying all the expenses. Spreadsheets should include a comp for market rent on the art/glass gallery, as I imagine you haven't actually been paying any, but the buyer will want to know how much rent it could achieve in today's market.
In addition to the broker's expertise, someone else who could be very helpful is the commercial lending officer in your local bank. You could complete a commercial loan application as if you were buying the building from yourself, then ask the loan officer how much the bank might be willing to lend a buyer assuming the applicant is qualified in all other respects.
As a rough rule of thumb, you could treat the $ value of the potential loan as 70%-75% of the price someone might be willing to pay for the property, given the 30%-25% balance of cash banks like to see from a buyer for this type of project, at least the ones around here.
You could also run a second set of numbers using projected B&B income rather than current income to see how much difference it could make to the Profit & Loss. This will also need to factor in the cost of servicing an additional loan to make any structural changes necessary for the conversion to B&B. .
However, bear in mind that if you ask someone to pay more because of potential gains, rather than realized gains, they have no leverage with the bank for that part of it. Hence you can only expect to achieve ambitious pricing if a highly qualified buyer with deep pockets can be convinced to buy into a dream, either the one that you are walking away from, or their own version. That's a tough sell, but it does happen and the great thing about a building like this, is that it's uniqueness will attract a lot of interest.
We purchased it for investment 7 years ago & my husband is now looking for less overhead.
Where are you going to advertise it? Why are you selling it? Was it family-owned? What is the overall condition? Check out http://www.loopnet.com see if that turns up anything.