I'll be honest--I thought the same thing for her parents.
But, their "middle" class setting (Translated into american, high upper middle class, I think)--doesn't allow for them to pay for the event required for royalty I don't think their estate is worth that much. And if it were, I hear the banks are cutting down on those HELOCs.

I also heard--the $40 million...is for a "low key" affair by royal standards. It is not expected to be the hoopla of Charles and Di. It will still be HUGE of course with pomp and circumstance--just not as much as they would do maybe in better economic times.
Also--I think taxpayers there complain like taxpayers here. They won't like paying for it as it is not their event.
However--heads of state from world wide will be invited and will show up. Nancy Reagan attended Chuck and Di's wedding--not sure if that was a logistical issue or a safety issue that President Reagan did not attend.
But our president will be invited.
In any case--even if many decline, or send alternates, many will still show up and that is just too much leadership under one roof. Recall the tragic plane crash of the Polish (was it?) govt several months ago. So many people on one plane from one government, gone. It is the DUTY of England to make every effort to protect those who do show up--and since it is a public affair (relatively speaking) tantamount to a state dinner or something, it does make sense for the taxpayers to pay for it. I would expect nothing less.