How much did a M1 Garand cost in World War 2?

A definitive answer to the value of any firearm is impossible without data such as barrel wear, overall condition, and potential part mismatches (several manufacturers made M1 parts). More information would be required to answer this question, and without an experienced, impartial opinion as to condition, only a general (and wide) range of values is available.

Also, if the rifle has any verifiable history (unit it was assigned to, specific soldier who carried it, etc.) it's value could double or triple.

Check to see if the Civilian Marksmanship Program has any M1 rifles for sale. They will generally provide a reference point for what a functional, but not necessarily collectible rifle is worth.