Really just depends on your site and how your choose to finish it. Development costs are dependent on where you place the structure relative to utilities, etc. What kind of finish are you looking for? Hardwoods or vinyl. We can custom build/design an ADU for approx. $50 per sq ft with a basic "contractor grade" finish.
There are huge swings depending on the quality of finish, type of construction, and whether or not you need to install a seperate septic drainfield- If you are looking for basic, basic ADU I bet you could find some designs in the 60-70$/sq.ft and it goes up from there.
You can do it cheaper but you'd have to get pretty creative.