Yeah, no way would I pay anywhere NEAR that much for a "Grady" standpipe
It's just a silly plastic tube. TH Marine all the way if you wanted to buy one.
However, if it's just the "screen" that's bad... I bet you could find something at Home Depot that you could make work pretty easily. Go to the bath plumbing aisle. Heck, even just a piece of galvanized screen that you form to fit in and secure with a little hot glue would work. Or even a PVC cap that you glue on (if it doesn't already fit snugly by itself) and drill holes in. Point is... this is easily solvable for just a dollar or two.