I have a/c and heat in my garage which is about 700 square feet in floor space. It is also open all the way to the peak of the roof which has a 6/12 slope. Walls are insulated as is the roof. My unit is a 3 ton Trane heatpump with the compressor outside and the air handler inside. The building is in the shade until about 10:30 and is back in the shade by about 4:00. The color of the roof is an off white/cream. I also have two large ceiling fans installed to help push the warm air down from the ceiling in the winter and to "stir up" the air in the warm weather which reduces the amount of time the a/c actually is needed. So far the highest power bill I have had for the shop was just over $30 for a month. The unit will bring up the temp inside from the mid 50s to 65 degrees in about 10-15 minutes or so. Same with the cooling side, going from the high 80s down to the upper 70s in just a few minutes. Cost for the install including all materials and labor was about $4k. Hopefully this will give you some numbers to compare against the twin wall units and their operating costs. Good luck with your project!
