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.
About £40 importing a copy of Relativity Visualised. I absolutely love it.
Then I spent about $90 getting a copy of the Red Wheelbarrow Mr Robot book. Im a huge fan of the series.