A $30M QB in 2019 would account for 15.1% of the salary cap, which would easily be the highest percentage ever on a team to win a superbowl in the modern NFL.
Superbowl winning QBs by % of salary cap:
Year Name Cap %
1994 Young 13.10%
1995 Aikman 6.70%
1996 Favre 10.20%
1997 Elway 5.20%
1998 Elway 5.00%
1999 Warner 1.30%
2000 Dilfer 1.60%
2001 Brady 0.47%
2002 Johnson 9.60%
2003 Brady 4.40%
2004 Brady 6.30%
2005 Roethlisberger 4.90%
2006 P. Manning 10.40%
2007 E. Manning 9.20%
2008 Roethlisberger 6.80%
2009 Brees 8.30%
2011 E. Manning 11.70%
2012 Flacco 6.60%
2013 Wilson 0.56%
2014 Brady 10.64%
2015 P. Manning 11.66%
2016 Brady 8.62%
2017 Foles 0.91%
2018 Brady 11.3%
2019 Mahomes 2.7%
1994-2017 Source