It's pretty hard to defend GW's pricing, to be fair. However, part of the reason might be the production capabilities available to them. Most kits are outsourced to China and thus cost far less to produce, whereas GW produces its own models in the UK where labour is far more expensive, and I believe Privateer use local American casting facilities where similar costs occur. Both are roughly comparable in price.
An Aliens or Zoids model has enough fan-backing to go into mass production, because it's an easy sell. You like the Xenomorph? Here, have Xenomorph. Their prospective customer base is far larger than GW could ever hope to attain, because GW require you to sit down and assemble your minis, paint them up nicely, and then get to display or play with them. To be fair, for the majority of us here, that's probably a major selling point.
I've often wondered about getting Revell models or similar, ground-based vehicles to splice into my Guard for conversions, but heck, it's difficult to find ones that look suitably comparable to what I'm trying to get them to represent, given the Guard's...unique vehicle design in terms of tanks.