From what I have heard, the regional was moved from the Sports Arena to the Forum for a variety of reasons, one of them being cost. However, in my talking with FIRST people, they said that due to the Forum being a union venue, the cost was very similar to the Sports Arena. (This may or may not be correct as I have seen nothing on paper, but this is what I have heard.)
I am waiting for the 2007 LA regional though. USC is building a new arena which will open in the summer of 2006 and according to the project website:
http://www.usc.edu/neighborhoods/galencenter/ community groups will be able to rent the venue at cost or for free depending on the event.