M/C $50s seem to be extremely common in the GTA these days. I've seen them on a regular basis since I got some crisp ones in May. From circulated bundles of $50s I'd estimate that there are probably 20-25% M/C notes.
I am personally hoarding a full bundle of AHN changeovers, but they're just so common that I question whether it's worth it.
I'd be surprised if we will see M/C $100s before the new polymer notes. I think those would be keepers.