MOST insurance policies do NOT cover hearing aides. As a person who's been wearing hearing aides for the last 30+ years, I can guarantee you this. Only if you work for a much larger corporation with a VERY nice benefits package, will you find an insurance policy that will cover your hearing aides - or even a portion of it.
My last pair cost me just shy of $4000. I paid out of pocket since my insurance at the time didn't cover this expense. This is, to date, the second biggest expense I've ever paid, after my car. They were top of the range 11 years ago. I can buy an equivalent model now with the same features from Costco's hearing center now for about $500 each.
Maybe your mum doesn't need the top of the range aides? Try looking for some with fewer features - say only six channels and two or three programs each (one program for normal environment, one for noisy environment, and one for telephone use if she should so desire). You'll save a ton of money.
The other reason why hearing aides are usually so expensive is that not everybody has the same ear shape. All in-the-ear aides are made from a custom mold, which does increase the cost. My dad recently got a behind-the-ear pair that didn't include a custom mold. The tips fit into the canal, similar to a pair of newer earbud headphones. (They still cost him $1200 for the pair though.)
Your mileage may vary. I highly suggest you shop around. Just remember though - you get what you pay for, and always buy the insurance plan on the li'l buggers.