in your query try this
where [isOnSale] is checkbox. if it's checked, [onSalePrice] will show [price]*0.9; if it's not checked, [onSalePrice] show nothing
Saleprice:iif([onSalePrice]= -1,[price]*0.9,0)
(if onsalprice is ticked multiply price by 0.9 else make it 0)