It's not really an upgrade. That's another one of those political things, I think. New engines, all new systems, new wings. Even a lot of the supposedly old bits have had to be re-worked to make them fit. In reality, there's very little left of the old airframes, & it would have been cheaper to have made 'em all from scratch. A lot of the cost increases have been due to delays caused by realising the old airframe sections they were going to have to to fit together with new CAM precision-engineered parts were decidedly non-standard .Hold on that article has the CVF's at 3 Billion pounds, yet the Upgrade of 12 nimrods aircraft to MR4 is costing 3.5 Billion pounds? If this is correct what a total screw up to put it politely, shows the priorities of the Gov and the massive mess, what a joke, if its bad reporting then fine, won't dont they screw the Nimrod upgrade order and get 12 P-8's I'm sure that wouldn't come in more than 1.5 Billion Pound for 12 would it???
The big money was on development, & has been spent. The marginal cost of each extra one, now that's been sorted out, isn't too bad - £1.1 billion to build 9, & bring the 3 development aircraft to production standard. And they have various advantages over the P-8. A pity they're not going to make more.
BTW, there was no P-8 when the decision to develop MRA4 was taken, so we couldn't have bought it. What we could have done was cancel the MRA4 after we'd spent most of the development money, & buy the P-8 off the drawing board, hoping the USA didn't cancel it, as they did the P-7, & accepting a 5 year delay.