I'm not sure if this will do what you are wanting but you might try this
Set the product price to 4.99
Create an order option like this
18" x 18" @4.99 /sq ft; *2.25
That will multiple the number of tiles that they enter in the quantity box by 2.25 to get the correct price per tile. The shopper will have to compute the number of tiles they need by themselves and then enter the number of tiles and not the square feet as the quantity.
If you want them to enter the number of square feet you could set the per tile price for the product cost and then the order option as
So product price = 11.23
order option
18 x 18 @4.99 /sq ft ; /.2.25
Note that this won't do any rounding so if they need 9 feet it won't come out right but you could have text to specify that they need to round their quantity to an even tile number of square feet.
You can see the other price modifiers available on
http://www.shopsite.com/help/10.1/en-US ... tions.html