I want to be able to turn on my gas logs automatically whenever an indoor
thermostat is calling for heat AND an outdoor thermostat says the
temperature is below a temperature selected from a range of 25 to 40
degrees F.

I am trying to find a low cost termostat for the outdoor portion of this
system. It can a simple mechanical thermostat with a SPST switch closed
when below set point or a SPDT switch.

Any pointers would be very much appreciated. I have not been able to find
anything on the web.

Thanks!