I have a stack of $2.00 Jefferson bills I purchased back on the first day of issue in 1976 and were taken to the US Post Office. There they placed a .13 cent postage stamp on the bills, and stamped the bills with a official cancellation stamp. The bills have been locked up since the first day of issue. How much would they be worth today?

Hi locman, this is information is from the 'AllExperts' site which I think may answer your question:

For the $2.00 notes the value is always questionable.
The 1976 $2.00 Federal Reserve Note with out a First Date of Issue Stamp is only collectable in Crisp Un-Circulated grade for about $3 to $4 dollars except for a couple of replacement notes with a Star in the serial number. In general terms the 1976 US Two Dollar bills are collectable since not that many stayed in circulation. Bills in less than crisp condition have less value and in heavily circulated condition only trade at face value.

A lot of these Two Dollar bills with the then NEW DESIGN were stamped with the first day of issue. Using the new 1976 stamp and canceled by the post office with a cancellation stamp design only used that one day. First Day Cover stamp collectors usually want them but many already have bought themselves one or two. Maybe you can find one who needs the San Francisco Post office cancellation? They would likely pay the most for the ones you have.

This is a cross-collectable made for stamp and paper money collectors.

The current stamp would be put on the bill or coin holder, and then canceled with a special U.S. Postal Service Cancellation Stamp, over the stamp with the date of issue being part of the cancellation stamp. Of course the cancellation stamp is only used that one day.

They are collectable! They may be a little more collectable than others.
But the price is still less than $8 or so since they had so many made and now the Bicentennial is more than 30 years ago. They did not sell them all when they were issued and the rest went at a discount.