Again, speaking as someone who has built a router from MDF (and, despite what I've said, it has turned out some useful work, but very slowly and with a lot of care needed to retune and adjust each time it's used), the real costs are in ballscrews, guide rails, motors and electronics. Your design using skate bearings and supported bearing surfaces for them will be cheaper, obviously, but at the cost of time and effort to build, adjust, and readjust. As Gerry said, it's easy to underestimate just how much force you need on those bearings to keep them in contact with the rails, which then need to be supported to stop them flexing, which makes it difficult to build the necessary box structure around them. The actual material cost for the structure is not that big a part of the overall build. The structure for my MDF machine probably cost around £100. The steel for a somewhat larger machine will cost, maybe, £300. The motors and electronics cost, maybe, £500, and ballscrews, profiled rails, etc, about £1200 (based on prices from China - local suppliers would cost a bit more). Decent water-cooled spindle around £300. So, the difference in going from MDF to steel adds around 10% to the overall cost, and will give a far superior machine.
OK, your numbers may be different depending on choice of leadscrews (trapezoidal are cheaper but have disadvantages) and guide rails (supported rails are cheaper than profiled rails, for example) but do the sums for the overall solution and compare the different approaches. If this is going to be a machine for a commercial operation, then factor in the general messing-about time for MDF, and costs associated with slower machining speeds.
And if you do still go with MDF or whatever, look at how you can incorporate things like steel angle along the edges of panels and steel box section reinforcing components to stiffen structures. Even bolting these in will improve the machine (mine now has a couple of steel box sections supporting the bed which has reduced sag enormously). I would still worry about the gantry for a 4' cutting width all the same.