The CMP heavily influences the M1 prices since they're the consistent seller of USGI Garands, so start there (url above).
Winchesters and IHCs generally run more due to collectors getting all wiggly about them (I have neither of these two manufs... yet).
Use the CMP pricing as a base (match the condition), and then anything you can point out about yours that sets it apart will bump the value.
If you want to get hard-core about it, buy into the GCA membership and start getting their (quarterly) newsletter.