As far as lowest cost? I think the zilla thermostat they sell at the petstores works (this one here?). It costs around 35 dollars, is nothing fancy, and won't make a person with a shiny new snake think "maybe I don't need a thermostat at all..." Problems with it are what you might expect; it tends to crap out after a few months of use, which is to be expected for buying the cheapest around. Reviews also say the dial isn't completely accurate and the temp you set will be higher or lower than what you get, so double checking temps is a must. Better than nothing, not as good as more expensive thermostats.
And for cheapest temp gun, I use a simple nat geo one that is also sold at the petstores (couple different brands depending where you go, 20-25 dollars on average). These are also considered "cheap" when compared to the fancy ones for sale at lowes that cost around 75 dollars each, but I would say that's only because it doesn't have all the bells and whistles (it tells you the surface temp in C or F, that's it). There aren't any specialty reptile shops near me, so big name pet stores have to do.