Hi Emma
I set my own practice up 3 years ago now. Autumn'08 I commenced the sub let of my premises to a reflexologist 2 days a week.
Naturally costing is dependant upon many things such as:
-How much you want to earn from the sub-let arrangement.
-Are you providing reception cover or not
-Will you allow use of your phone, computer etc.
Naturally there are things you need to consider and these will vary as to whether you are on site or not. It's a good idea to come up with an agreement which is signed between you. This may not be entirely legally binding unless you involve solicitors but at least you will have a level of understanding between you.
Things you need to consider are:
-Will the reflexologist be 'Assocation Trained' (Just peace of mind to you that they are appropriately trained and professional)
-Confidentiality and data protection
-Ensuring your premises are used for only that which has been agreed upon
-Cleaning responsibilities after use of premises
-Security (locking up after a session if you're not on site. etc.)
-Ensuring the respect of the premises and trust of the sub-lettee (as chiropodists we not only have medicaments which can be dangerous if mishandled but there is other aspect of having quite a lot of stock on premises at any one time.
I know it can sound a little daunting

however if you take all the necessary steps you can work happily and healthily alongside a sub-lettee.
From my own personal experience it has been really positive:
-Extra income during difficult times of the current economic climate (I charge £70 per day)
-My practice is now open 5 days instead of 3 (better visibility)
-The reflexologist has her own client base (opportunity for me to gain some of her clients and vice versa)
-Another professional to bounce and feed off in terms of practice management and marketing.
Hope this is helpful Emma
Good luck in your pursuit
Sharon