Return on investment in a spa depends on many factors, inlcuding:
What kind of spa (day spa, resort spa, etc.)
How well-managed the operation is
The amount of debt service
That said, a typical rule of thumb is that the more the spent on the build-out, the longer the amount of time before break-even is reached. So a small day spa may take a year to stabilize, whereas a large resort spa may take 3+ years.
A healthy return in the spa industry is 25-30%. However, many spa operators hover around 10-12% and consider themselves lucky.
The spa industry is not a market you enter for "the buck." It is a labor-intensive, front-line customer service business that takes both heart and business skills.