How much interest should a business pay to finance air conditioning?

I need a/c for a dog training center - 3,500 sq. ft. - in Maryland. My landlord is an HVAC contractor.

Will your landlord install the HVAC, then cover the cost as a part of your leasehold improvements in your monthly lease payment? If you are a tenant, the AC becomes a part of the building. When your lease expires and you don't renew, you will not be able to take the AC with you if it is built in and if there is no stipulation about ownership in the lease agreement.
Figure the average useful life of a good quality central air system at about 15 years. If you have three years to go on your lease, pay 3/15 or one fifth of the cost plus interest. Commercial loan rates vary. In my area, 6% is a fair interest rate on capital improvements.
You should also consider getting an estimate from a contractor of your own choosing. Your landlord could say the cost is $8,000 where an independent contractor might estimate $6,000.

You should pay the least amount of interest that you can. Call around to banks and credit unions to see what the prevailing interest rates are.