Excluding textbooks for classes, I bought this for ~$300 (several several years ago as a special treat)
Most of my collection is thrifted/bought from garage sales (for a quarter to a dollar usually) but I do have a handful of nicer ones.
Yeah my encyclopaedia eorzea books were like £35 each iirc and the art book I have was similar. But in terms of normal books I don't think I've spent more than like £15 ( brand new stormlight books so chunky hardback)