If Walder Frey was more of a risk taker before the Red Wedding I think it would feel a bit more comfortably plausible. I'm probably forgetting something but I don't think anyone considered the Lannister grasp on the Iron Throne stable at that point. Walder delivers up an the last major force on the field that can threaten the Lannisters on a silver platter for ... Riverrun and maybe a few marriages. At the cost of utterly ruining the family name for generations assuming everything goes well or the utter destruction of his family if things don't go so well. I'm half tempted to reread the books but on reflection it feels like he's selling too cheap considering the risk\reward.
The Red Wedding works for a better story but I guess the essence of my quibble is that it doesn't fit logically with the character or even the established setting. Gods and magic are real in Westeros and the evidence of such is thick on the ground moreso than in our historical world... 700 foot tall walls, dragons only a few generations in the past, Atlantis level apocalypses, and folktales that probably have their origin in the not too distant past... but very few (if any I think if you look closey) of the major players are particularly superstitious or even religious. Breaking the salt bond is a sacred oath... murdering guests invites the worst wrath of the gods\Fate and not just men and Walder crosses that line for a few fees and marriages in a world where superstitious claptrap is real.
Heh, this is probably an area for me to put on the blinders but it does sort of break the quasi-medieval setting for me considering how weakly held faith and religion seem to be in the series.
There are seven kingdoms in Westeros - the North, where Winterfell is the seat. Even with the castle razed, it still has symbolic power and is the reason why Bolton holds on to it despite the weather in A Dance with Dragons. Riverrun is the seat of power in the Riverlands - whoever controls it, controls that kingdom. The Riverlands are well populated (unlike Dorne and the Stormlands), and wealthy, though were just devastated by war. So, controlling all of the Riverlands through Riverrun is a pretty big prize.