Off the top of my head I can't remember the exact reasoning we used to get the 75-150gp figure in the FRCS, but it's based on the average pay of a common laborer: 1 sp.
I think it went something like this:
You own a slave for a year, that's 365 days of work, or effectively 36.5 gp worth of money you've saved by owning a slave instead of hiring a peasant. For two years, that's 73 gp worth of work. Now, you have to subtract a slight amount from that "mony saved" value because you have to provide food, clothing, and shelter for them (things they would provide for themselves if they were employees instead of slaves).
Depending upon how you treat the slave they may live for only a few years or more than twenty. Setting the slave price at 75-150 gp is a good range, is simple, and prevents people being worth less than a longsword (15 gp) or work animal (light horse, 75 gp) while not making them so expensive that only the most rich people can afford them.