The cash value of B prospects

In recent years, Littlefield has signed quality veterans at low prices and traded them mid-season. Last year he was able to acquire Bobby Hill, Freddy Sanchez, and Ray Sadler in trades involving Kenny Lofton, Jeff Suppan, and Randall Simon. Here’s my question: what is the cash value of a B prospect? How does that value factor into the decision to make veteran trade-bait signings? If we assume that Raul Modesi was signed as trade bait, the Pirates invest $1.75M in him to get (a) some playing time from Mondesi, which has value, plus, perhaps, (b) the addition of a prospect mid-season. These days any first-round pick gets a $1M signing bonus, and most third-round picks get a $500K signing bonus. If Littlefield targets a AA or AAA player, then he’s targeting players who have, in most cases, benefitted from several years of additional nuturing investment (salary, instruction, uniforms, hot dogs, etc.). Is it not easy to imagine that the addition of a prospect like Bobby Hill or Freddy Sanchez is worth $500K or $1M or $2M in cash? Each time a prospect advances a level, he moves that much farther from the value set by the signing bonus. Those skills must have additional value. Anyone know how these guys are valued and care to share some non-proprietary details?