Attic insulation over garage

I live in Texas. (108F yesterday) there is no insulation above the garage in the attic. We have a mini gym and workout area. Due to dogs we usually have the garage doors down. If I add insulation (about 400sqft.) How much of a difference will I see in temperature? Is it worth $400 to insulate it myself. The neighbor said to insulate the garage doors. Any advice would help. Thanks