An ACDELCO part #12570260 Flex fuel fuel composition sensor sells for $338.89 plus shipping at www.rockauto.com
Just for a comparison; I would phone the parts department at a Chevy dealership and find out how much they charge for a flex fuel sensor for your truck.
It looks like this part, shown here: http://www.rockauto.com/catalog/moreinf
o... is easy to change.
After the part is replaced; the code which triggered the check engine light must be erased from the computer's memory with a code scanner. That will turn the light off and restore normal vehicle operation.