Interesting para towards the end of this piece on the USD3 billion quoted for the 50 helicopters (USD60 million a pop)
UK requests remanufacture of Apaches to AH-64E standard - IHS Jane's 360
Remanufacture, which involves stripping out many of the helicopter's systems and cross-decking them into newly-built airframes, should be the cheaper of the three options as these parts have already been paid for. However, the DSCA notification puts the unit cost of the remanufactured platforms at USD60 million each, which is three-times the value previously given by Boeing for new-build helicopters. While the reasons for this discrepancy are unclear, it could be that the DSCA figure does not factor in the savings that would be made as part of the US Army multi-year buy, which would likely have to be negotiated after the UK's request is approved. If this is the case and the UK does join such a procurement plan, its buy should then turn out to be closer to USD1.5 billion (not including sustainment and support).
(and a less powerful engine than the RTM322?)
Without Googling it, it's my understanding that the engine for the AH-64E is as powerful as the RTM322 (although both are limited by the helicopter's transmission anyhow). Happy to be corrected on that though.