You are right on about the cost and time really on these companies...either they don't have their act together, are too small, cater to too many other makes/models, just reaming the $$ for some 'name' they came up with...
Once you have the 'engineering' done, you make the prototype and then the tooling (mostly just jigs/fixtures), order your materials; cut; trim; prepare; set-up; weld; finishing touches; paint/coat; box; ship, etc...Computer modelling makes it nice to get your cut dimensions, factoring in shrinkage and all that, so to me a pair of sliders should be $300-350 (for a company that sells a fair amount and is set-up properly) and that's still giving some profit. The more you make, the less, although welding time is welding time, so that does not change, but materials/expendables in bulk do go down with more as does set-up time...Course I'm making this sound easy, but I'm also being honest with seeing how things work in the fab/manufacturing places I've been and am currently at, as well as being involved with the stuff.
However, to be fair, I would also keep in mind supply and demand and IF a product is top quality, I could see a little extra $$ added to it and there's just not enough of a market to really get too interested in making these, it seems.
I understand the welding part may be an obstacle to somebody wanting to make their own, but if you can design what you want, take dimension, cut and prepare your materials, you could then take them to a welder, along with a drawing of what you want and it shouldn't cost a whole lot.