$400-500 is about right for ground up basic finishes with no surprises. Separate design if you need it, I think. Do you need it? You may find an architect to make the plans who is a designer at heart, and you can embark on a creative journey( cue orchestral music).
If it's a small garage conversion there is a path to do it cheaper -get a draftsman to do your plans and submit your permits- and just be creative. If it's just a little garage try DIY owner/builder for the basics and bring in a good finishing crew at the end with a designer friend.
I agree with @Bruce Woodruff plans first...though plans are expensive so I think you are smart to explore here. Between plans, and engineering, and geotechnical, and a survey, AND permit fees ect ect groundup you could spend 15k easily. Then you're ready for to interview a contractor. After all that time it may even be cheaper to build. One guess, for what it's worth, at this rate is construction may cool off by Oct/Nov and you may be able to build at a discount- in perfect time for your plans to be completed. Good luck.