coop board asks for 13 moths of cc fees

It'd be very unusual for a co-op's governing documents to prohibit a sublet fee. You might want to check yours.

It's usually some percentage of the maintenance, and your 8.33% isn't out of line.

If the board is imposing that fee on only you, and not on everybody who sublets, then that's another story.

From my experience... that is actually VERY CHEAP!!!
Consider yourself lucky!

This seems in line with the policies of many coops, though usually it's a rental fee of some kind. This actually seems quite reasonable.

And new policy eliminating ongoing subletting in mairad's building in five, four, three, two ... ONE!!!

Wow. Your building is allowing you to sublet "for years" ... with NO fee ... and is now charging you only 8.33%.

And you're COMPLAINING??

you can try get yourself voted onto the board to change the policy however as per comments above,
1/ yes its legal
2/ its comparatively cheap.