You should be taking the temp right on the glass -- you want to know the warmest temp that the snake could possibley be exposed to so it will not get burned. I would say 95 would be the very warmest you want the glass to get. With that temp, depending on your substrate, it should be around 90 on top of the substrate.

You could use one thermostat for both cages, given that the heat source for each is exactly the same, and they are set up the same (you probably could run all three cages if all variables are the same).

If they are set up differently, you would need to get two thermostats, or something like a Herpstat II which can contol two things. Herpstats are expensive, but reliable. Reptitemp 500r is cheap, but then you run more of a risk of failure.

I don't know for sure, but I would think thermostats would cost about the same online as they would at a show. I don't think there is much room for bartering like you could with livestock.

You can pick up a Herpstat direct from Spyder robotics for about $100 (or around $200 for a Herpstat II). I picked up a Reptitemp off of Amazon for just a little over $25 including shipping.

Good luck!