...Things come in lots of bigger sizes. Vehicles, tanks, buildings, mecha, portable shields whatever. Its all the same in the end. And in MFD, its pretty much simplified as far as I can get away with (you'll do much better following the Brikwars vehicle rules).
...These rules are built around the idea that the Big Thing is already built and that you are trying to come up with a cost for it. Don't try to start with the cost then build the Big Thing! Build it first. After a few tries, you will get the feel for how much your thing will cost while you are building it, so you can judge better while building.
...I had already done some things up for Big Things, but I realized they were getting complicated and too far from what MFD was supposed to be about, so here's the fixed and improved version.
MFD 214 - Step 4 : How Much Damage can a
Big Thing Take
MFD 215 - Step 5 : What Happens when a Big
Thing Gets Hit
This is the simple way of computing Big Things costs and handling Big Things in the game. These rules are great for throw together games when you want to guestimate how many figs you have to pull from your squads to replace them with a Big Thing.