Here's the problem you're running into - the X2 mills weigh about 150 pounds and use iron castings made in China, right? So there's not much metal there, it's cheap metal, and a lot of features can be cast or machined into the castings. Now compare that to routers - routers are bigger than an X2 (or at least the one you want), so if you made the parts with cast iron, they would have to be more massive to prevent them from deflecting under their own weight. This would make them cost a lot to ship from China among other issues. So most people here (and most kits / plans you will find) use materials that you can buy and assemble yourself, with aluminum extrusion being pretty popular. Without getting too crazy, 1018 steel is about $1.75/lb, while 6061 aluminum is about $7.14/lb, and most routers in this size range would probably weigh the same or more than the X2 (I know my gantry alone weighs about 140 pounds, but mine is a bit beefy and a bit bigger than you're looking for). So right there you can estimate that the frame is going to cost more. Now add in that because you're using extrusion, you typically have to add machined blocks to support the screws and linear bearings plus the linear bearings and rails (since they're not machined into the castings) plus a lot of fasteners. That all adds cost. So no, it's probably not really likely that you'll find a frame design or kit that costs under $500 for the size you mention, and honestly I'd be suspicious of the performance of any design that was that cheap.
The typical way people get cost down is to build with wood or scavenge materials, but both have drawbacks. They can be overcome to some degree at the cost of your time and ingenuity. Maybe check out Joe's cnc - that might get you closer although the kits are not complete afaik, so you'd have to do some searching here to find total frame costs. I think they also use some mdf and hdpe. Link: Kits & Plans