...to relay the signal. Add under 100 to the total cost.
More than likely though, if it is a frame built house without significant plaster in the walls...locating the access point in the center of the house will solve any distance issues you may have. (unless your house is...say...200 yards long)
Drywall(sheetrock) is not a significant barrier. However, if you're in a house constructed in the European style (concrete or cinder walls) then you will have a problem with wireless and will need to set up AP relays.