There is a good sale that end on April 30th. $30 less is not too bad for what the card as to offer.
You got to be sure that you’re unit can run those cards. There is a document to verify that:
http://www.navionics.com/en/compatibility
Looks like you need the 648P for lake Ontario
http://www.navionics.com/en/platinum-plus-1#coverage
This is some examples of what you can get:
