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.
Im looking at World Book sets on ebay. Brand new, they go for around a grand. So im looking at ebay for older sets, they run a couple hundred it seems. Gonna get a set for the house for my kid to use.
My husband got me the Game of Thrones book series a couple years ago, I dont think theyre cheap, but most books I have cost less than $20.