I'm having the same problem with the outer edge. Looking at US forums, many suggest low tyre pressure, but others say general wear on steering components. Interestingly, many on that forum say that 10K for the front tyres on such a heavy car is acceptable. The quality of the tyre will make a huge difference, budgets do wear very quickly, and that's what mine has, but because they were on the car when I bought it I couldn't justify replacing them as they were fairly new. The fact that you've had two sets wear in the same place does maybe suggest wear on the steering/suspension somewhere.
Have a look at this, it may help
http://www.clublexus.com/forums/ls430/550584-excessive-tire-wear.html