Doesn't count against the luxury tax, and the relative cost is small compared to the potential benefit, every prospect you develop, even high picks, can save you $10-20M a year compared to filling that role with a veteran FA.
I'm not talking about guys like Miller, but players like Kemp, or young RPs who you might be able to turn from replacement level or below to 1-2 WAR players. Turn one or two marginal prospects into solid major league depth or decent starters and you get a huge return on your investment.