That's a tough question to answer. A lot depends on what old Lawn Boys are selling for in your area. There are people who love them as trimmer mowers or ditch cutters and there are people who like them for their simplicity and light weight. And then there are people like me that just like them for a whole lot of reasons. A nice clean mower that doesn't look like it went through a war will obviously sell for a better price. Post it on Craigslist along with nice photos and an accurate description and you might be surprised at what you can get. Start high and see how much interest it generates. One thing I notice about Craigslist--nice photos make all the difference. If it doesn't move, re-list it for a lower price.
A mower like yours, in nice condition, might bring $80 where I live, but that is just a rough guess.