Are you accounting for the AIrbnb service fee? Guests get charged not only your fee, but a fee that Airbnb charges for all transactions.
To see what guest would pay to stay at your place for any certain days, just go to your listing as if you were the guest (go to manage listing, then "view" on right side) and then enter the dates guest wants, as if you were the guest staying at your place. You will see a breakdown of standard pricing. That is the same amount that the guest sees. Then, just replace the standard accomodations amount you see there with the special offer you are giving, and consider that Airbnb service fee will change slightly as well since it is a percentage of the total you charge.
