Was Hammond underpaying Nedry for his work?

Nedry was a contractor. He agreed to a price for the project and then felt it was beyond the scope of what he had bid. Hammond held him to the agreed upon price and Nedry resented that.

Nah. Him screwing up the security system, turning off the fences, and then planning to leave the island make him a no good stinking jerk. I don't feel one bit bad for him when the diloposaurus kills him.

He stole from the company, not a great employee

Nedry is a probable addict who likely gambled or overspent everything he got. Very overweight people are frequently addicts in multiple domains, and their addictions are often very destructive and poorly managed.

He likely bid low on the job because he was immensely talented (another result of his addictive tendencies) but often unconscientious, which led to his disastrous escape attempt and whatever previous mistakes Hammond was referring to.

The “evil capitalist who ruins everything” is quite stale at this point, might you agree? Even if Hammond had paid everyone through the roof, things could still have gone wrong in the exact same way if Dodgson was able to offer more (and the inciting incident had nothing to do with worker’s compensation).

He couldn't have been underpaid if Hammond was paying him what he agreed to do when he bid on the work.

What I don't get is Hammond always yammering on about how he "spared no expense." Yet he hired a single guy for all of his automation work. This is incredibly incompetent on so many levels.

Let's put aside the questionable character of Nedry and assume for a minute that Hammond hires a respectable vendor for the work, but it's only one guy. What happens if the one single guy has a heart attack? What happens if he gets hit by a truck? What happens if he wins the lottery and walks away with his finger in the air? So much can go wrong.

Awarding a contract is about more than just the money. It's about the value you get for the money. You'd pay more for a reputable vendor with a history of good work and a staff with support and safeguards than you would for an individual person, but it's worth the investment. Putting it in context with "spared no expense" makes it even more of a foley that he awarded the contract to Nedry - trying to pinch pennies with the lowest bidder, instead of hiring a proper vendor with full support.