This is in essence a very difficult question to answer. A guestement around £1m to £2m depending on a lot of varibles. As Cyrano says its not so much a direct cost but the cost of the infrastructure that is required to be put into place. But you would need to proportion out the cost of that infrastructure. Take an ATR you need it to be able to get an AOC but what percentage of the operating cost would you put towards the certificate only. Even the cost required by the CAA are variable, SRG outline a fee based on x number of hours to say approve manuals. Go over that time and cost rise. The suggestion of hosting on an AOC is in theory a lot cheaper but the CAA in the UK(not inrespect of IOM etc) frown against that with that simple answer. You want to operate an ATR in your name then you need your own AOC. As they say been there seen it done it.
XT