The charity shops I go to range from £1.25 - 3.50. I tend to prefer to just go on amazon and buy a book for a penny and then pay the £2.80 packaging, mostly second hand but a fair few have been new. I get shed loads of cheap books from amazon, I will never understand why people pay full price from big book shops.
Ask in your local public library when they are to have their next book sale to get rid of their more tired volumes. They will be cheap. But if you live in Oxford there is a charity in the Market Place (Gloucester Green ) which is giving away books to save them from being pulped.
Charity shop prices increased when they started selling only those books that show little in the way of wear and tear. These days they dump the rest. Round my way the Red Cross shop is the cheapest for books. But Eccles, have you looked at the 'Project Gutenburg' website? Over 30,000 books to download to your Kindle for free. Admittedly, they're out-of-copyright books, but do include nearly all of the classics.
i think oxfam expensive anyway and not just for books I know at one time ( not sure about now ) oxfam used to pay their managers a quite high wage, whereas other charity shop workers work for nothing. I usually find car boots much better for buying books i can get really good condition ones 6 for a pound. when i have finished with them i give them to charity shops so they benifit a little.