Do Real Estate Agents charge commission on seller paid closing costs?

I was wondering if its standard practice for a seller to pay real estate commissions on the amount paid for buyers closing costs that is built into the sales price.

I was an escrow officer ages ago and I dont remember this being standard practice, however I have a real estate agent which has built $6000 into the price of the sale of my home for buyers closing cost and has calculated his commission based upon the sales price + this $6000

Any idea if this is the standard practice?