It's asking because certain closing costs are added to the purchase price to determine your cost basis, which is used to determine any gain. If it was your primary residence and your are eligible to exclude the gain without it (or you have a loss) , you can enter zero. If the program doesn't want to accept 0.00, then enter 1.00 If you have a taxable gain on the sale you would want to include the costs to reduce your gain.
