To begin, elections, in total, are now costing in the billions of dollars. Overtime, this cost has increased. Midterm elections are not as expensive as presidential elections, but looking back at the past 9 elections, here is the estimated cost for each one:
Each race is becoming more and more about outraising the opponent. If the candidate has money, then they are able to increase the amount of ads, campaign information, hire a bigger team for voter outreach, etc.
However, if you ask Bob Healey, who ran for Governor of Rhode Island, he only spent $35. Take a look at his campaign ad, “A New Hope”: