OEM block heaters are a lot less than $75. TSC often gives the illusion of lower prices. Sometimes yes, sometimes not so much.
I never had very good luck with the Thermostart, Rick. I use 15W40 Chevron Delo 400 and even with a fully charged battery my 4610 was hard to start with the Thermostart unit at 20F. It seemed like it didn't turn over very fast. I attribute that to the cold oil viscosity requiring more torque. With a block heater it fires right up on the first rev. Maybe my starter is weak.