I have yet to find any computer controlled thermostats (for an HVAC) that are less than $300 and aren't ugly as sin or extremely limited in functionality or interface. After research how thermostat controllers work I realized a simple microcontroller communicating via UART can control a few solid state relays via GPIOs. For fall-back (non computer) control, I decided to add an 20×2 character LCD display and 4 buttons to control the temperature.
The low-power nature of msp430 chip, the solid state relays, and LCD display means decent battery life can be expected out of two AA batteries. Power might be derived from the HVAC control wires themselves, though I can't recall how many miliamps are provided.
This is part of my largely custom Home Automation project.