The largest price differentiation for a service isn't the cost of product but rather your time. The cheapest solution compared to the most expensive may save you only £1-£2 per application so reducing your price by anything greater than that savings without reducing your time is a false economy as the only thing cheaper would be you!
Everyone is different and clients all have different needs. However I would rather spend the extra quid on a quality product that will look great and fade off evenly rather than have my best advertisements walking around with a patchy, fugly end result just to shave a little off cost per service. Put another way... if you spent the extra £1 in a better quality product and it yielded you one more client because someone saw and loved the result... that is worth far more to you as a professional!
That doesn't mean that more expensive is better. It is just to give perspective to your thought process.
There are a lot of really great formulations out there to try with a wide variety of price ranges, but would strongly suggest you sample them first to find the one that performs for you and your clients FIRST, price second! We do MoroccanTan and are having great success with it because of its approach to formulation, but it is a premium brand designed for performance first. Feel free to drop me a conversation (PM) if interested in giving it a whirl.
Best of luck to you!