There probably is no standard because the T.I. allowance is so integral to the overall lease attractiveness. Doing some research on your competition is the best way to find out how aggressive you need to be when negotiating that point. Depending on your spaces' needs you may not have to give much. Standard allowances include HVAC, bathroom, and an allowance for finishes. The value of these items would need to be ascertained by a general contractor. I would be happy to help.