.
The largest allowed dimensions are 4 ft tall by 6 ft wide, which is 48 in tall and 72 in wide.
The smallest allowed dimensions are 3 ft tall by 5 ft wide, which is 36 in tall and 60 in wide.
Two things are obvious:
1. Than smaller the total size of the final mosaic is, than it is cheaper.
2. Than greater the number of large tiles is, than the mosaic is cheaper.
Therefore, the cheapest solution is this:
Place 6 x 5 large tiles (6 vertically and 5 horizontally) to cover the rectangle of the size
6*6 = 36 inches vertically
and 12*5 = 60 inches horizontally.
All conditions will be satisfied, and the cost will be minimal, which is OBVIOUS.
So, you need 6*5 = 30 large tiles (and 0 small tiles).
The minimal cost will be $4.50*6*5 = $135.