Condition isn't everything, it's just mostly everything. Without a description of its bluing, wood finish, wood quality, wear signs, bore condition, etc., it's impossible to say. A "beater" gun with marred wood and well worn blue, but with an action that is in good condition, is probably worth about $400 for the value of the action as the basis of a custom rifle. The value goes up from there as the condition goes up. Naturally, its value is hurt if it is unoriginal (cut stock, buttplate replaced with pad, sight hood missing, etc.)
There were only about 10,000 or so of the L57's made, and it appears there were fewer .308's than .243's (and even far fewer .244's). A significant number of these 10,000 L57's also went to produce non-Sako guns sold as Colts, Ward's, and various custom guns (in Canada, a rifle called the "North American Grizzly" was marketed on the L57 action.) And many of the 10,000 were sold in countries other than the U.S. A few of those imported to the U.S. have been lost or destroyed for various reasons. So, just how many L57 .308's are floating around out there? Hard to say, but in the grand scheme of things, not all that many.
Bottom line: Sakos don't have a widespread universe of collectors like those for Winchesters or even older Marlins. So assigning a value to the relatively scarce L57 .308 due to its small numbers isn't necessarily an exact science. My opinion is that an L57 .308 is more valuable, condition being equal, than it successor L579, which is in turn a bit more valuable than its successors. How much more depends solely on the buyer and the seller.