If you listened to the "buy it off the shelf as it is, from the yanks" crowd, then our AH-64s would certainly have been cheaper. But they'd also have been a deal less useful in Afghanistan, and the price would all have been in $, with little or no benefit to the UK economy, and the exchequer would have recovered nothing in tax, as it has done from WHL and from the WHL workers who built the WAH-64Ds.
And while US manufacturers sometimes produce better kit, American isn't always best. S-92 vs Merlin, for example.