Among the Democrats, Jared Polis could be the biggest spender. He has spent whatever it takes to win all the primaries he’s ever competed in (estimated in 2018: $3 to $5 million). Ed Perlmutter is probably the frontrunner and much of the Democratic establishment’s candidate. As a seated congressman, he can call on significant D.C. interests to support him ($2 to $4 million). Noel Ginsburg has funds and personal money that could be spent ($1 to $2 million). Mike Johnston could likely raise funds from individual sources ($1- $3 million).
On the Republican side, Walker Stapleton has major out-of-state sources from the Bush network ($2 to $4 million). Victor Mitchell has indicated he will spend personal money to win ($2 to $4 million). Kent Thiry, who would try to appeal to independent voters (and may run as an Independent), could invest whatever needed ($5 to $7 million). Jack Graham spent $1 million last time and will probably need to commit more this time ($2 to $3 million).
This partial list of guesstimates indicates a low of $18 million and a high of $30 million could be spent in the 2018 gubernatorial primary.