It's not an expense, it's a capital adjustment.
If you are the buyer, and you pay $100,000 plus a $5000 commission, you would enter the cost basis of the property as $105,000. By including the commission in the cost basis, it may reduce your capital gains if and when you sell, but it is not an expense deductible against ongoing income.
If you are the seller, and you sell the property for $100,000 but you pay a commission of $5,000, then your selling price is considered to be $95,000. This reduces your capital gains tax, but is not an expense deductible against current income.
