The pro detector range looks a bit out of my price bracket, too. But the first two on the page should be affordable to someone with a really keen interest in bats, shouldn't they? And the last 4 are no more than some people would pay for a new lamp to replace their old stinky.
I saw a bat in Aygill Caverns last week, I used my eyes and detected it, even without the cost of contact lenses to correct my defective vision I reckon I would have still spotted it; costly detectors, pah!
More seriously though, probably great for someone (or a group) with a very keen interest in recording bat numbers, average person probably not so.