I'm a long-term volunteer in a charity shop (an Oxfam Bookshop, so we don't sell clothes, though we do sell donated CDs and DVDs.)
To echo what some other people have said:
I respect the fact that some people feel that some charity shops are overpriced and up to a point, I can agree. The deputy manager in my shop said that generally, the guideline for our specific shop is that we must mark books at around a third of the original price. If it's practically brand new (i.e. in perfect condition and released within the last twelve months) they want us to mark it at 50% of the original price.
I actually do think this is a bit too steep and I personally think that it should be 25% of the original price for older items and perhaps 40% for very recent releases.
However, in certain shops, the days of getting an item for 50p or one pound are gone. The charities do not see themselves as there to help those of us with low incomes get really cheap things (and to be fair, that is not their official role.) Perhaps in past decades, charities did see helping those with low-incomes as an additional, unofficial factor, but if so, that line of thinking has changed - they are now out entirely for their specific cause.
Some people might argue that lowering prices (at least on certain items) to £1 might increase sales and bring in more money overall that way...perhaps that would be the case in some shops, but it really wouldn't work for everywhere across the board. Sometimes, things just won't sell even at £1. And if they did, it relies on a consistent stream of at least average-quality items to instantly replenish the ones sold - the quantity of donations, as well as the quality, can often fluctuate. People have donated books in the past that have literally been falling apart. If a paperback novel is priced at £2.99 or £3.99, then only one copy has to sell in order for the shop to get that 3 or 4 pounds, whereas the opposite is true if we priced them at £1.
There is a lot of pressure from above to try to meet targets. Honestly, even if prices were reduced, I'm not sure if shops would make our targets. My shop manager feels very pressurised and you can tell it affects her. Why, just this last Christmas they expected the shop to be open for the full 8 hours on Christmas Eve, when prior practice has always been to close early (at around 3 or 3.30 pm.) That's apparently what our manager ended up doing, but it was in violation of what she'd been told to do.
Our manager sometimes gets told off by her area manager if the shop ends up being closed on certain days. The aim is for us to be open 7 days a week, but the organisation that collects our culled stock has a habit of going for weeks without showing up, which means that in our basement crates of uncollected scrap piles up, as well as donations. I'm part of the team that volunteers on Sundays (both of our managers have that day off, so it's the one day when all the staff in the shop are entirely volunteers) and when things get too congested in the basement, we feel it's unsafe, so we refuse to show up. Our manager sometimes gets told off when this happens, even though it's entirely out of her hands.