That's nothing, I'm -10 in both eyes and some people on here are worse. My glasses don't cost anywhere near £400 though, more like £75.
My eyesight is rubbish at -7 and -6 (with a big side order of astigmatism on both). I don't have clear vision at all - if it's close enough to see, it's too close to focus on. -10 must be truly dire.
I got my current glasses as barter for helping an optician out with a computer problem (very generous of him - it wasn't a lot of work and they are polycarbonate lenses) but I could get a new pair for under £50, maybe as low as £20. If the OP's dad is being told he needs to pay £400 for glasses because he has a "high" prescription of -3, he's being ripped off.
Agreed.If you need glasses then get some.
OP: Your shortsightedness may well get worse over time and stabilise at some point in the future. That's the way it works. It's not caused by wearing glasses. All you're doing by not wearing them is stopping yourself seeing properly.