How do you figure commercial building rental costs. I am trying to figure how much a rental space will cost?

If I have the rent how do I figure how much it is per square ft. and if I have the square ft cost how do I figure how much it is per month.

Depending on the type of space you are renting you might find that buildings like warehouses, and retail units charge by the square foot or the yard. Usually the newer build properties or the highly sought after locations do this. You will need to find out from the owner or leasing company just what that figure is for the amount of yardage you need.
On the other hand- depending on the business and your location, you will have to pay 'in going' costs to take over a lease. This could run into the thousands! Not all businesses of course require it- but some do- especially restaurants!
But to be perfectly honest- this is a question that you will have to take up with the owner/leaser of whatever space you want to rent. There are dozens of factors that will make up your final costs. And they are the only person that can give you realistic figures. Hope this helps and I'm not just rambling on! Good luck in your venture anyway. :)