In my master bedroom the toilet is located on one end in a 5' x 4' room of its own. My shower is located on the other end in a corner about 2.5' x 2.5. I want to swith them and make that toilet room a shower. How hard and expensive will this be? My home is on a slab foundation with a septic tank. Please help.

There are way too many variables in this question for a remotely accurate guess be made. Your best bet is to get an on-site estimate. I'm sure somebody will pipe up with a "ballpark" estimate, but lets not forget how big ballparks are...

To install shower in place of toilet: not a big deal. All you need is to convert toilet pipe into 2" drain and bring in hot water.

To convert shower into toilet is a big deal. Shower has only 2" drain. You need at least 3" drain for toilet. You will need to cut the slab, find closest 3" or 4" sewer and connect to it.

Milo just touched on the problems you'll face in this remodel./ Expect the slab to be jackhammered up, cement dust everwhere. The 4" toilet drain line will have to come up and a new 4" line cut in and ran to the new location. Milo simplifies the shower conversion. If yo reduce down to 2" off the toilet drain line you will need to run a vent up to the attic and revent into a dry roof vent. Note: If you live on the West Coast and fall under UPC Pkunbinh Code the toilet may already be vented.
Be prepared for a big mess and some heay labor expense.
Still want to swap? Good luck in whatever you decide. Tom