I had most of mine given second-hand to me by my father, but the cheapest I have seen bought for me was the WITCHES TRILOGY omnibus. It contained the first three Witches books, EQUAL RITES, WYRD SISTERS and WITCHES ABROAD (currently reading Witches Abroad again). It was this price along with GOING POSTAL and NIGHT WATCH because they had been damaged in transit.
My book Unseen ~Academicals cost £4 for the paperback, £7 for the hardback.