I would first create a list of different facials you can perform with the products you are buying in.
People like a choice, especially when it comes to the price.
So I would say you need to have a list of 5 to 7 different facials. As the price goes up, more gets included in the treatment.
For example, your cheapest facial could be a cleanse & mask (30 mins mini facial) for £x
Then your next one could be a basic facial (Cleanse, exfoliate, massage, mask) for £x + £10
Then a more expensive one could be an Essential oil facial (cleanse, exfoliate, essential oil massage of face, neck, decollete & arms, mask) £x + £20
and so on....
Think of some clever names to make each facial sound special. Include the brand name if it is a famous one.
For initial promotion, definitely launch them at the price you want them to be in 2 years time and then run promotions for limited time periods.
As you are already established with nail clients, I would run a promotion that combines a facial with a nail service for a discounted price. This should get you up and running with them.