Mine were pricey, a few years back now but definitely more than £3000. It may have been as much as £5000. This was in 2006/7.
In my case I had to have the sides of some teeth ground down to make room for the movement. Really unpleasant. Also "buttons" on some teeth, little tooth-coloured lumps, to help the traction of certain teeth which had to be rotated. And my dentist also added some elastic bands for good measure at one stage! (Disclaimer: my dentist is an absolute perfectionist!)
Oh yes, and before he even started on Invisalign I had to get all of my metal fillings replaced with white ones. Which cost a fortune. I still don't know if that was just my dentist, he did say some were in a bad way, but of course my teeth do look better for it.
The liners themselves were fine, everyone at work knew about them because a colleague in the States had them before me, but people who didn't know me didn't guess.
Having to go back and forth to the dentist very frequently for short appointments was a PITA. Luckily I had a flexible job close to the dentist and could pop out easily.
When I got pregnant with DC1 I was 95% done but couldn't stand the liners and retainers any more as they made me gag, so that was that. My dentist put a retainer wire behind my top teeth, the main ones he'd straightened, and I've had that ever since. My teeth do look much better compared to old photos.
I so wish I'd had a brace as a girl like my sister - her treatment was free under the NHS and so much less hassle!
Apparently there is now some cheaper system that promises results in a shorter timeframe for less extreme cases?
Personally if I could do it again I'd have a word with myself for spending so much money on my teeth and would probably just have opted for train tracks (you can now get clear ones) over a shorter period of time.