If a 744 costs £10k/hr, then a highly bespoke single-purpose machine at £45k an hour isn't so expensive IMHO.
However, this entire debate is meaningless. The AH64 program costs £x. Not £y per use. I'm sure AS90 costs a damn sight less per round, and surely for £46k one could have rather a lot of AS90 rounds on the ground. But then it wouldn't be doing the same job. For £46k, one could have a truckload of grenades, but not doing the same job.
If the country is fighting two wars simultaneously, and failing to learn the lessons of history (see Nazi Germany for a good pointer), one of which is in a country where we got our arses kicked despite having infintely superior kit, and where the Russians got their arses kicked despite having an entire army on task, then least we can do is pay for it!!!!
Edited to add: Or rather, the least YOU can do is pay for it, since my taxes go to Uncle Sam these days, and that's a whole different issue.