Depending on condition, and WHICH version of the M77, between $250 and $500.
The Ruger website shows that serial number range to be late 1974 to early 1975.
If the Ruger website is up, it has sn data.
400-800 or so
It is a RUGER Model 77. That is what the M77 means. Typically the M77 is the "older" style with a tang safety, blued with walnut furniture.
250-550 USD
100-500 or so
200-450 USD
$350-$550 depending on wear. More with good optics.
Ruger has detailed sn data on their web site.
$100-250, depending on condition.
$100-250, depending on condition.
50-250 usd