A single, medium-sized metallic asteroid with a mass of around 10^17 kg, if composed of pure iron (it isn't--metallic asteroids also contain much more valuable materials such as nickel, magnesium, gold, platinum, palladium, titanium, ruthenium, silver, uranium, chromium, rhodium, etc. Keep in mind, the vast majority of metals in Earth's crust were delivered by asteroid impacts, after the planet's surface solidified) would have a gross value (assuming the price of raw iron ore is $4/kg) of approximately $4 quadrillion US dollars (that's four million billion dollars). Of course this would flood the market with metal and drive the price of iron and other metals down to nearly nothing. The value would still be in the hundreds of billions or even trillions. A single asteroid, put into low Earth orbit and systematically mined, could pay for NASA's entire combined and cumulative cost throughout its entire existence, several hundred times over.
This would make building materials much cheaper, making housing more affordable. Manufacturing metal items would be much cheaper, leading to more availability of high technology in developing areas. With a surplus of metal there would be little to no need to strip mine our own planet, leading to a cleaner environment and more preservation of natural beauty.
Consider also the possibility of orbiting solar farms, that can face the sun 14 hours out of every day (or more, or variably more and less, depending on orbit). The energy could then be beamed down to anyplace on earth by microwave laser to a collecting station. This would be about five times more efficient than terrestrial based solar panels, infinitely cleaner than fossil fuels, and without the risks of nuclear power.
Claiming that space exploration is worthless is one of the stupidest things a person can say.