The cost and difficulty of space greatly exceeds the cost and difficulty of crossing the Atlantic.
The basic problem is that you never ACTUALLY run out of stuff on Earth - you just start getting ore concentrations that are unprofitable to mine, but that you could get whatever metal you want out of if you wanted to (or out of seawater.)
It’s just always going to be cheaper, if the supply of something gets short, to just go after low-concentration ores on Earth (plus recycling) rather than asteroids.
All that you say is true with today’s technology. But who knows what the state of the art — in space as well as on earth— will be in 30 years.
As I mentioned before, in just the last five years the fracking revolution has added another 50-100 trillion dollars worth of oil/gas reserves to the USA alone.