It's difficult to give an off the cuff figure, but the region of £millions comes to mind.
In the first instance you have to form the company, get approval from the CAA to operate, source you aircraft, lease them and pay a bond up front, employ a few but very expensive specialists to recruit and train pilots, cabin crews and engineers, do your marketing and constantly monitor what the competition is doing.
All that has to be done before you have earnt a bean.
Just take our word for it, even for the smallest of operations, it's very seriously expensive and even in the best economic environments, few attempt it and even fewer succeed.