I put one of the HPI Baja kits together last winter. Very nice kit. I went brushless. The kit was $750 before any radio gear or power was added. Radio gear and power was another 400 or so bucks. I already had batts and a transmitter/rec. Very happy with this car so far. Pic of it in my sig.
With that said, I agree with CobraGT. The Rovan Baja ready to go with gas power is $499 shipped. All you need is gas/oil mix and some batts for the transmitter. I have a 30.5cc Rovan baja and it has been 100% good to go so far. Only a broken screw so far and I hit a curb pretty good when it broke. I am still impressed with it. I thought I was going to have a bunch of problems with it and just have not so far.
Parts for the Baja 5b are all over the place. HPI, Rovan and King Motor are all well supported.