Cubs are made by MTD, so it's still an MTD, for better or worse I don't know anything about these.
But I had to repair a rusted-out deck on my Craftsman, it was something of a process. It took me a while with the welder, and I was fortunate enough that the rust-through was just on a vertical wall, so the patch piece was easier. Yours is also on the "corner", so patching that would require a more complex shape. Mine was a rectangle, clamped to the wall to make it match the curve, and then welded in-place. And I also had to reinforce it at a hanger mount.
I don't have much context for what these normally sell for. But if it's a homeowner-grade ZTR (which is my guess), then I probably wouldn't want to pay $1000, knowing that at a minimum, I'll be spending a bunch of time on deck repairs. To say nothing of potentially worn transmissions, etc.