Even GHQ micorarmor tanks are fairly inexpensive, when compared to larger scales. I think they're $9 USD for 5, and the quality is great. If you really need dozens and dozens, and are inclined to do so, making a 1:285 or 1:300 model tank wouldn't be that hard. You can sculpt in sculpey clay or in greenstuff or even balsa wood. There are online tutorials for making simple molds using RTV silicone.
While it is possible, unless you're in need of lots and lots of vehicles, and have lots of time, it is probably not cost effective to do so. When you include the cost of moldmaking supplies, release agents, emergency room visits (just kidding), and such, your real cost will likely end up being a substantial fraction of the cost of professionally cast figures.
Unless you're a remarkable artist, your sculpt will not be up to the level of detail that GHQ, CINC, or even Irregular can muster.
Now, there is a certain coolness to casting your own figures, and I've done it and am richer for it in a personal wholeness sense. You'll walk taller than other men of the same height. Women will find themselves attracted to you in a way even they won't understand. It is the same effect you get when you make your own home-made pie crust. In a pocketbook sense, though, it is near lunacy unless it is something you cannot buy elsewhere for a reasonable price.
Irregular miniatures makes among the most inexpensive 6mm tanks here: irregularminiatures.co.uk/.
Their tanks are .5 GBP or about .80 USD each, plus shipping.
There is a company called xeno games that sells axis-and-and allies style plastic pieces. Hasbro may sell pieces as well as replacement components. Eagle games sells them, too at this link link
I once made a bunch from wood scraps. The trucks and half-tracks came out well, the tanks were a bit blocky.
Good luck with your project.
Jim