Errata ....

I wrote:
=IF(C2 <= 6, 0,
MIN(10,K2)*17+MAX(0,K2-10)*33+MAX(0,K2-30)*50)

That should be simply:

=IF(C2 <= 6, 0, K2*17+MAX(0,K2-10)*33+MAX(0,K2-30)*50)