I hate to break it to you but there is no way you can build a 12 vdc 100 plus amp linear supply for anything even close to the cost of a switcher, it cannot be done unless you have a surplus transformer, diodes and caps. Further, you haven't even considered regulation. How close do you think you can get with off-the-shelf transformer voltages? News flash, you can't, it needs to be custom if you're looking to do an unregulated supply.
The bottom line is the difference required in raw materials, copper, iron and capacitance and it is is quite significant when you're dealing with 60hz (linear) verses 50khz or more (switcher).
BTW, don't count on Chinese switchers to delivery their "rated" output current, most don't. This is one of those things that if you try to go cheap, you'll probably end up paying twice. Once for for what you thought would work, a second time for what you really needed.