It's very difficult to answer this question directly, mostly because that the costs you or I would face when undertaking such an endeavour are primarily capital in nature. For a large airline (or other organisation), they'd have staff and equipment on hand already, and the price an organisation like BA or BAE pay for parts is almost certainly different to the list price.
That being said, the deal in question is notoriously bad value for money. From that source, the taxpayer will be paying around £10.5 billion for 14 Airbus A330 troop transport/tanker jets with a capital value of only about £1 billion, although the deal also includes maintenance. Although converted to military specification, the converted passenger jets will not be flying into a war zone any time soon.
Interestingly, it appears that the government itself didn't really know the answer to 'how much will this cost', even as they were signing the contract to provide new aircraft. Specifically, a critical report from the National Audit Office concluded that there was 'poor access to full cost data – the Department never gained visibility of detailed sub-contractor costs and margins for the aircraft and their modification'.
In other words, nobody knows!