Not really a bad month to have.
Aug 68 through May 69 are most common.
June & July 69 appear less often (normal slow down for model changes).
As for what they are worth, you will see them all over the board.
Some guys prefer to find them with 1.94 valves anyway and have them machined. Also, many guys will want to upgrade to stainless valves, and will just ASSUME they need guides.
I don't know anyone who would buy a pair that was rebuilt in the 70's and try bolting them on.
If they haven't been surfaced, and still have the press in studs, $200 each and up, depending on whether someone is "hot" for the particular date.
http://www.ebay.com/itm/1969-1970-70-Cor...872&vxp=mtr