I recently removed my thermostat while painting in my home, upon completion I wired the thermostat like it was originally but now it seems I am not getting any power to the thermostat or the unit outside. I checked the wires twice but there was not problem, checked the breaker outside but it had not tripped, check the breaker in the house it wasn't tripped, then hard wired red, green, and yellow wires together (advised by HVAC tech) but it the unit did cut on. The only thing I can think of is that a wire may have touched and shorted out the transformer or something else. Has anyone had a problem like this with a 1987 Janitrol AC/Heat Pump system?

Yep, if you didn't remove the power before you started removing wires you either fried the transformer or if you have a circuit board there is an auto type blade fuse that blew.

Is the transformer costly to replace or is that something that should be fairly cheap and easy to repair?

Not hard to replace. Remember to disconnect power before making repairs. Not expensive either. Just make sure you replace with the same size. Someone on here said you could get at Radio Shack. Also can find at HVAC parts house or online as well. Consider putting an inline fuse with the installation as well. Much cheaper to replace a fuse than transformer