We have a 24' round 13500 gal pool and would like to know if a 28 gal tank will do the trick. Can you give me a ball park figure of how much that size tank would cost?

The amount of propane a tank can deliver is directly related to the surface area of the liquid propane inside the tank. My understanding is that almost all portable tanks are NOT large enough to deliver enough propane to operate a gas pool/spa heater.

Yep, Dr_Spa is correct. You would empty a tank pretty darn quick and they are not designed to give you the volume of LP needed for a heater. Can you access NG? Have you looked at heaters yet, they will tell you the size of line needed to provide fuel and some can be switched from LP to NG and bac, but not all.