how many 8' tiles are needed to tile a 8' by 18' floor? how much money will it cost if tiles are $2.75 each?

I will assume that you have a typo, and that you meant 8 inch tiles, not 8 ft tiles

so the floor is 96 inches by 216 inches
so you will need 12 times on the short side and 27 for the long side.
or 324 tiles, for a cost of $891.00