Is 84 million a lot? Yes. Guaranteed? YES.
However, that is the going rate and the state of the league for a starting QB that is average to above average (remove Blake Bortles, Case Keenum, Tyrod Taylor, etc. who are in the $15-$20 million a year range.) Every QB contract signed for anybody who is just barely a starting QB will always be the biggest contract ever signed at that point in time, until the next guy signs.
2013: Joe Flacco, 6 years, $120.6 million
2013: Aaron Rodgers, 5 years, $110 million, $54 million guaranteed
2013: Tony Romo, 6 years, $108 million, $55 million guaranteed
2013: Matt Ryan, 5 yaers, $104 million, $59 million guaranteed
2014: Jay Cutler, 7 years, $127 million
2014: Colin Kaepernick, 6 years, $126 million, $61 million guaranteed
2014: Andy Dalton, 6 years, $115 million
2016: Andrew Luck 6 years, $140 million, $87 million guaranteed
2016: Joe Flacco, 3 years, $67 million extension onto 2012 deal
2017: Derek Carr, 5 years, $125 million, $70 million guaranteed
2017: Matthew Stafford, 5 years, $135 million,
2018: Jimmy Garoppolo, 5 years, $137.5 million, $74 million guaranteed
2018: Kirk Cousins, 3 years, $84 million, all guaranteed
2018: Matt Ryan, 5 years, $150 million, $100 million guaranteed
2018: Aaron Rodgers, 4 years, $134 million, $100 million guaranteed
Point being, yeah Cousins has a big contract, but it falls exactly in line with any other deal of a starting QB. Unless you can get a top 5 draft pick and a franchise QB, which isn’t happening to the Vikings in the next few years, then they probably made the best move that they could make to get a QB in signing Cousins.