This may be the case if you are offering the client a cost-neutral selection or if you are giving the option of a base package that you included in the contract.
If you haven't entered an allowance amount, the most common way of handling this is to put a price of $0 for the choice. This does not mean that the item is free to you, but rather that choosing it causes no change to the final price of the home. Any future upgrade options you offer should have prices that reflect just the incremental upcharge to the client.
Another way to handle this is to enter an allowance amount, and then enter the same price for the choice. For example, you may have included a $5,000 appliance package in a home. You could then enter a predefined option of “Standard appliance package” with a price of $5,000. The price and allowance cancel each other out, resulting in no change to the final price of the home. Any future upgrade options you offer should show the full price, from which the allowance will be subtracted.