I'll go through this with you one step at a time.
For the same price, NASA could have SpaceX build and launch ten rockets.
That would be ten times as many scientific experiments launched or whatever good thing the rocket is doing.
Alternatively, they could spend the same money to have SpaceX build ten rockets, then throw nine away and launch one.
That's virtually exactly the same as what they're doing - taking billions of dollars from taxpayers and ending up with one rocket.
It ends up exactly the same as throwing away nine rockets, removing from the economy whatever value those nine rockets have.
They could also spend the same amount of money having SpaceX build one rocket, then spending a few billion dollars sending kids to college. So, for the same price you can have either a) one rocket built by NASA or b) one rocket built by SpaceX plus provide a college education for a thousand people.
So what's the difference between those two? Both add a rocket to the economy. The difference is whether people get a college education or not. Which do you think is better for the economy, college graduates earning good money, or those same people flipping burgers? Waste REMOVES value from the economy.