Asking the Forces that had joined NPAS for the FY 2013/14, FOI requests have given me the following information:
Total paid for Air Support said Forces for the full FY prior to them joining NPAS: £36,087,787
Total Revenue Contributions for FY 2013/14: £27,778,351
Total Revenue Contributions for FY 2014/15: £32,657,594
On top of that are various Capital Payments, in addition to the Revenue Payments, which for 2013/14 were some £9,000,000. These are deducted from Forces annual Home Office grant.
I asked the same Forces how much the were paying for the current FY. Eight Forces failed to reply or didn't know, but of those who did reply, the vast majority said that they were paying the same as last year.
So, not sure where all these savings are occurring, but it doesn't seem to be at individual Force level.
It will be interesting to see what they pay next year when there are on 15 aircraft left.