This was the 2nd time Gore had violated the rule so that's why fine is high.
http://espn.go.com/nfl/playoffs/2012/story/_/id/8873686/2013-nfl-playoffs-frank-gore-san-francisco-49ers-fined-105k-low-socks-source-says
I think the fines being previously agreed upon and set by NFLPA in the CBA is too hard of a concept for some people to grasp, given the comments I've seen (in other places).
NFL was giving a fine based on what they're able to do, if there wasn't a pre-set amount then Goddell probably would have wanted to Bankrupt Brady (and everyone else that does questionable stuff).
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Now back to the regularly scheduled trolling.
Brady was fined less because he only used one leg to kick, while Gore wore his socks down on both legs. We all know two is greater than one, hence the larger fine.