Just to go into further detail:
Backlund: $3.5 million AAV, 53 points in 81 games with 63.7% defensive zone starts
Sobotka: $3.5 million AAV, 33 points in 61 games with 50% defensive zone starts in his last full season
Staal: $3.5 million AAV, 65 points in 82 games with 50% defensive zone starts
Cullen: $1 million AAV, 31 points in 72 games with 64.2% defensive zone starts
Jarnkrok: $2 million AAV, 31 points in 81 games with 54.7% defensive zone starts
Henrique: $4 million AAV, 40 points in 80 games with 51.6% defensive zone starts (he had 50 points last year too)
Cizikas: $3.35 million AAV, 25 points in 59 games with 67.4% defensive zone starts
Smith: $3.25 million AAV, 32 points in 74 games with 51% defensive zone starts (he was at 36 points in 81 games with 58.4% defensive zone starts in 2015-2016)
Berglund: $3.85 million AAV, 34 points in 82 games with 58.5% defensive zone starts
Namestnikov: $1.95 million AAV, 35 points in 80 games with 46.9% defensive zone starts in 2015-2016
Bozak: $4.2 million AAV, 55 points in 78 games with 47.5% defensive zone starts
Haula: $2.75 million AAV, 34 points in 76 games with 64.7% defensive zone starts in 2015-2016
Eakin: $3.85 million AAV, 35 points in 82 games with 52.6% defensive zone starts in 2015-2016
With arbitrarily not including young players on ELCs or bridge deals, you can still come up with a lot of guys who put up similar to better numbers than Bonino in defensive roles while making less money. And here are some comparables for Dumoulin:
Larsson: $4.167 million AAV, 19 points in 79 games last year
Hjalmarsson: $4 million AAV, 18 points in 73 games last year
Pesce: $4.025 million AAV, 20 points in 82 games last year
Despres: $3.7 million AAV, 23 points in 75 games in the year he signed that deal (cap was also lower at that time)
Ekholm: $3.75 million AAV, 18 points in 80 games the year before he signed that deal (signed it in October of 2015)
Dumoulin isn't overpaid. He's just flat out not.