If you are cost conscious, then an aluminum plate is your best bet. However the whole idea of a steel plate is to move more weight to your back and off your weight belt. So in most cases steel is more appropriate. (Not always, just in most cases.)
For 15 ft of webbing you won't pay more than $20.
For 4 D-rings and keepers, and a steel belt buckle, another $50 at most.
The most expensive part will be the wing itself. Oxycheque has good prices on these, around $350 each in various types and styles. Oxycheq has cams built into their tank straps, which hold the tank in place, and so you don't really need an STA, which saves you about $150.
Sure, you may have only saved yourself about $50, but the satisfaction of building your own gear is priceless. And you did save.