Commercial real estate contracts do not have the sort of "customary" costs, which are more commonly associated with residential home sales (such as a 50/50 split of closing costs, title insurance, etc.). The only "customary" cost is the real estate sales commission, which is usually in the 10% range, rather than the 6% for residential sales.
In my business dealings, I have always advised clients to sell the property on terms that require the buyer to pay all closing costs (except for pro rata property taxes). That way, there's nothing to negotiate. You are selling property X for $Y, and you're not interested in the collateral costs, because you're not paying any of them. The buyer will have to counteroffer based upon what he/she believes is affordable -- but you will know exactly what amount you are getting net from the deal, because you're not paying the costs, no matter what costs are charged, and by whom.
Concerning interest rates, a large institutional lender is likely to give a commercial borrower a rate, close to prime, which is currently 3.50% -- on a building, but only if the building is likely to generate high rents. However, as a seller carrying a note for a borrower, you are competing not with the prime rate (which the borrower undoubtedly cannot get because if he/she could, he/she wouldn't be borrowing from you) -- but rather with "hard money" private lenders, who typically charge 2.5 times the prime -- or more. So, for this issue, my answer is that 2.5 X 3.5 = 8.75% is extremely reasonable for seller financing of a commercial property.
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