"If taxpayers spend $50,000 to send a kid to college, will his lifetime earnings increase enough to recoup that $50,000 in increased income taxes later on?"
You are ignoring the time value of money.
Compare the net present value of the money spent now to send a kid to college to the higher taxes the educated kid will pay later in life.
Even accounting for inflationary pressures, it's still a dramatic net positive. According to this study (http://www.ericdigests.org/2003-3/value.htm), the average kid with a high school diploma in the US can expect to earn about $1.2 million over the course of his lifetime. A four year degree increases that number to $2.1 million. At a 15%-20% tax rate, this means improved tax receipts of $135-180k over that persons lifetime. Unless inflationary pressures explode, that's a net gain for the country no matter how you look at it.
And again, keep in mind that the rest of that income increase will be spent buying goods and services that will boost the rest of the economy too.