It's not just that simple, or cheap :(
Here's a personal totting-up for you, then...
I don't work in a church, but I do work at an institution with a chapel attached, so that'll have to do. We also have a more general use lecture hall, which has proper built-in radio mic equipment. Between the chapel and random other places around the site, there's a few other instances of similar but more limited and mobile equipment. As it happens, before reading this I've been online today looking for some spare parts because one of the cheaper bits (an all-in-one clip-on mic) has bought the farm. That alone is sixty quid. We don't buy crap - but it's not top end either. However, I'd certainly rate its lifetime in decades.
The built in rigs, which we have two of, are over FIVE HUNDRED POUNDS (for the mic, transmitter, receiver, power supply, cabling... so call it £450 for the bits we'd have to change). God only knows how much the other bits are, but I'd reckon at least a hundred each for five or six items, and they'd be total losses as the transmitters are built into the handheld mics. We can ill afford replacing any equipment that we don't need to; we've spent several grand in the last few days that we weren't expecting to have to do, and it's pretty much crippled our AV budget for much of the rest of the year. So what do we do here?
What we will likely do is just keep on using it blindly, as we have the problem-turned-benefit of industrial strength walls that are basically faraday cages. Hardly anything gets in or out. For the first time ever, that's a bonus. That or play with the frequency adjuster until it's up on the stops and most of the new interference disappears.
(I'm hoping we're actually dodging the bullet, as the stated frequency range is 830 to 866mhz... but as 855 is in the middle of that, and is what at least one of the units defaults to, I'm not so sure)
Now if we can take at least another thousand similarly medium-small cheese, up and coming but eternally cash strapped - and more so for the credit crunch and all the mucking-about with our usual funding - institutions around the country...... well that's easily a cool million without even touching a "real" church.
If we assume each church and it's hall has some of the cheaper but perfectly good quality items we have (a couple at maybe £100 a throw), then that's still a considerable bill. There's a LOT of churches out there.
.... and DO, DO PLEASE remember that this equipment isn't just used for services. Any community event that goes on in the church hall and its grounds (and one local to me of which I'm not even a congregation member - my faith lapsed a while back - hosts a great many events which I occasionally attend) will be affected.
All this so we can have yet more, ever more niche TV and radio channels. There's only so many that an island of 60 million can viably support, and only so much choice that any one person needs in this regard. For everything else, there's maste..... er, the internet. Or the rental shop.