S.H.
It really varies out there and per state.
There are 2 ways:
1) Interior Designer: they have Architectural education/training/knowledge. They can, draw Blue-Prints, lighting plans, electrical plans, do space-planning, and basically build a home, building permits, coordinating the project and Sub-Contractors as needed, and the specs for it from the ground up etc. But their focus is the interior. Versus an Architect, is the exterior. Some do both.
2) Interior Decorator: they do 'decorating.' Not structural work. They also do "space planning", color coordination. Decorative approaches.
So, each specialty, has different costs.
AND if the Professional works for a firm... then that varies also, versus if the person is an independent contractor.
Per hour or a cost per "project" is usually what it is.
BUT, know that in this business, things can often go on and on, for longer than originally thought. Because, there are SO many changes of ideas... per the client. ALL along the way. So then, costs can increase. As is typical in this business.
Bear in mind, in this business, the 'Client' often changes their mind... thus increasing the time-cost of the Professional... and the end date of the finished work.
Be specific in what you want.
Then you can tell that to whomever you call.
ie:
1) Space planning
2) color coordination
3) lighting
4) actual 'remodeling' or not, in which case this requires renovation work or breaking down of rooms/walls.
5) just help with accessorizing
And yes, you can utilize a College kid, majoring in Interior Design.
And maybe then can get credit for it, at school.
BUT, bear in mind the Prof will have to guide them, unless the student already has real-world experience.