That is very tricky.
Your most successful spas run about half services, half retail. They both generate about the same $$ per square foot, but retail takes up comparatively little space and one employee has a nearly infiinte capacity to sell, where as one technician can only generate $X per day in services.
Essentially, retail is a 50% margin game.
So if you go with an affordable line, then 50% of affordable does not put a whole lot in your pocket at the end of the day.
Plus, it ties up money for as long as the stuff sits on your shelves.
50% of high end is more attractive, but only if you can afford to have several thousand dollars tied up.
Then you have to factor in sales tax management - if applicable in your state -- one more form to fill each month and liability to manage - even if you sell nothing.