10G over fiber / SFP+ is cheaper than 10G over copper, because it requires lower power to drive a laser signalling through fiber than it does to drive the circuitry and enough signal through copper. Also, 10G over fiber has seen a lot more use over the years and there used to be a lot more development on fiber equipment for 10G
You can maybe find used older 10Gbps SFP+ switches even cheaper on eBay, but those probably have active cooling.
Theoretically, there exist SFP+ RJ45 10GBaseT transceivers that allow you to connect to an SFP+ switch with copper, but those have limited distance because of not enough power (hp makes 30m ones).