A large part of the reason for the wide payoff differential is that, no matter where you attend, the cost of law school is steep.
Median Initial Earnings and Total Debt
The data for median (i.e., 50% above, 50% below) initial debt and salary after law school summarized below from NALP for 2007 demonstrate the significantly higher payoff differential for upper-tier law schools (a lot more earning power without substantially more debt).
Intro Repayment*Assumes no increase over three-year period. However, tuition for 2010-2011 rose significantly, and further increases should be expected.
**Average of schools ranked 26-102; if schools in US News' third and fourth tier are included, the average three-year tuition is approximately $96,847.
These earnings and debt statistics from 2007 show that even before the current downturn, the earnings potential, debt spreads were significantly higher for graduates of top 10 law schools than for others, and were significantly higher for graduates from schools ranked 11-25 than for those of "below 25" schools. Market contraction means that it is very likely that these spreads will increase in the next few years.
Source: http://www.advisein.com/law-school-costs