What is a Speed Sensor and what should repair cost be?

You need more information. there are all kinds of speed sensors, Vehicle speed sensor for the computer, runs about 25 bucks on Amazon and is very easy to replace, takes about a half an hour to do. others are the ABS speed sensors, more complex work, roughly 2 hours of labor, they run about 20 bucks per unit. Sounds like your shops labor rate is a little high, as well as the part prices. I would go for a second opinion at a different independent shop. Or maybe there is someone out there just looking for some weekend work that knows his stuff.

They didn't do a diagnostics they read the codes , It would be worth it to have a good garage take care of it . The yhave $1,000's worth of diagnostic tools . And you need quality OEM/dealer parts/sensors installed . DO NOT buy junk parts from auto zoo , Oh Reallys or even nipa

Suggestion....take the vehicle back for another code scan. Write down the Numeric description of said codes. Get back to us with result.

I agree, the codes will tell us which speed sensor it is. It's probably the vehicle speed sensor since the check engine light is on and not the ABS light. If your speedometer is not working or not accurate that also points to the vehicle speed sensor. Jim