Pretty hard to say without more information and pics. It could be worth anywhere from 250-550 depending on the shape it's in & wear. I assume it's the tang safety model? If so it could be worth more because a LOT of people (including me) highly prefer the tang safety model over the m77 mk2.
just did a google search (just because i wanted to see what one looked like) and these two auctions on gun broker were the first two hits:
one with a beat up stock that was used- they started the bidding at 475
one "liberty model" -made in the bicentennial year(1976), that was brand spankin new (factory test fired only in original box with the bolt in it's own smaller box and rings in a little red bag that was tied to the trigger guard and the original manual) that they wanted the starting bid to be 799.
sadly the bidding had ended and no one bought either one they look like nice guns. hope that helps you.
2 years ago I got an M77 Tang safety. Bought it for $125.00 Put new bottom metal in it and stoned the trigger. Shot it for a month or 2, and then someone had to have it. I sold it for $250 with a $20 scope on it.
It does have the tang saftey, and it is in mint condition. I cant pst pics because I don't own a digital camera. I have another in 30-06, I changed the stock to a bell and carlson because the wood stock on it had cracked.