Whenever you advance a number of class levels equal to 3 times your current LA, you are eligible to reduce your LA by 1. It costs (current ECL-1)x1000 xp. Your LA is then reduced by 1. If it isn't reduced to 0, when you again advance a number of class levels equal to 3 times your current LA, you are again eligble (same cost).
Examples:
Drow LA +2
Eligible for reduction at class level 6 and 9.
At level 6 it costs 7000xp ((6 class level +2 LA -1)x1000xp)
At level 9 it costs 9000xp ((9 class levels +1 LA -1)x1000xp)
Half-Celestial LA +4
Eligible for reducation at class level 12, 21, 27 and 30.
At level 12 it costs 15000xp ((12 CL + 4 LA -1)x1000xp)
at level 21 it costs 23000xp ((21 CL + 3 LA - 1)x1000xp)
at level 27 it costs 28000xp ((27 CL + 2 LA - 1)x1000xp)
at level 30 it costs 30000xp ((30 CL + 1 LA - 1)x1000xp)
At our current level, you will only be able to have reduced your LA to +1, you need to get to ECL 31 (30 class levels plus the +1 LA) to reduce it that one last time.