I've done a few DIY projects, but I'm not sure whether designing with acoustics in mind is always less costly. I think the end result will definitely turn out better when you're starting from scratch rather than solving problems later. For instance, if you have a small band and you want to practice at night, you'll know to use dense flooring and to use noise controlling materials between the walls. You'll know the best placement for the electrical outlets and everything else. Overall, designing with acoustics in mind should give you a better experience, but I'm not sure if it's cheaper though.