It bluebooks between 250 and 600 (600 for collectors quality mint condition) if you're talking about the 300 discontinued in 1972, with any rust, not over 400 I'd guess, but that's a theory. In reality, with air guns in Canada, it's a buyers market, and I wouldn't expect to get that much for it. Only the states takes collecting arms that seriously, especially fringe items like air rifles.
There's also the 300s discontinued in 1996 though, if you're talking about that model, it's PAL rated so you might get closer to blue book which is remarkably similar.