Would it be cost effective to insulate around the outside of a slab house, and what materials should be used?

If you mean below grade, the frost wall. Probably and under your slab. Probably Extruded Polystyrene.

But where are you? How deep will your foundation/frost wall go? What kind of foundation do you have? or are considering?

Yes the frost wall which is 3' deep. The house was built in 1986 and i can only assume that is 8" thick concrete.

So you are considering excavating the frost-wall to insulate it? If so, no it is not worth it. If it was new construction or exposed for another reason, yes it would be worth it.

Really? I have access to a backhoe so the only expense I would have is the cost of materials and fuel.

By doing so, how much do you think it would raise the temp. of the slab?

I don't know about raising the temp of your slab but friend of mine has done this for a full basement. He had water intrusion problem and since he was already digging it up, put in 2'' eps. By doing so he cut humidity by 50% inthe basement. As for being your last house or not, this could help as a selling point in the futur and in the meanwhile you'll be much comftable inside.
Cheers!

It would be cheaper to do it on the inside if the house is already built.

What do you mean? Yes this is already constructed.

If it's a slab, there is no way to insulate the frost wall from the inside. I suppose you could rip up all your floor covering, (tile, wood etc), and subfloor, add insulation, then put down a new subfloor and a finished floor and shorten all the doors, but if he's got a backhoe and can use it it seems a lot easier to do it on the outside. And a lot less disruptive of the life going on in the house.

I think the thing to ask is how much fuel will he save vs how much work and materials he'll put in to the job.

Do you know for sure it's not insulated? Have you dug up a bit to see?

Oh ya, it's not insulated, had to dig up the water line once.

That would help in the summer because it does sweat.