I paid about $9 shipped for my ELM237 from Amazon and went with the cheapest one that had decent reviews. This was several years ago.
Today I just picked up the
LG Optimus Exceed 2 from Bestbuy for $15. Sometimes it goes on sale for $10. Regular price is $20. It should make for an excellent Torque display that I can leave in the car. The display is 4.5", which is bigger than the iPhone 5.
For less than $25, I've got a very capable gauge that includes various battery and inverter temperatures with alarms, RPM and power outputs for both MG1 and MG2, among many other things.
Nothing will overheat without me knowing, and I'll have better insight into how the Prius operates under various conditions. If the Prius were a 70s commercial jet, I would be the engineer. Way more interesting than piloting a cattle wagon.