Just found myself in this exact position with my B3030, so I called my dealer.
He said about $120 from Kubota. Ouch.
I only needed the back screw (implement side), but that's not available separately. Only the tightening nut.
So I picked up a heavy duty CAT 1 toplink at TSC for $49. Looks pretty beefy. Now the only thing is that it won't fit onto the tractor side. The new one is too wide. So I'll either have to grind down the sides or return it.
Edit: Note, TSC did have the "standard duty" version for ~$23. But I liked the HD version -- it looks a lot stronger than the regular one.