One of the better cardboard miniatures I've thrown together for prototyping was combining some plastic bases from Warhammer and Warhammer 40k, smooth head thumbtacks, and some thicker cardstock, cardboard, or foamboard.

For your standard mobile suit or humanoid character, first you print off pictures of your units with a white background in vertical rectangles roughly 0.75 x 1.5 inches. Gundamn has tons of resources around the net for nice full art profile shots of the various mobile suits and support vehicles.

Next cut out your characters and using a glue stick or whatever, stick them to your cardstock/cardboard/foamboard and cut them out again. You'll have a nice firm piece of art.

Now take those plastic bases and punch the tacks through the bottoms so the pointy end is now sticking out of the top in the center. You can guess the rest, but take your character pieces and stab them onto the point and just like that you have a nice looking do it yourself miniature.

The base really helps make the pieces look more professional and clean then other options I've seen, but you can print out smaller bits of paper with quick reference stats, line of sight reminders, or whatever and glue/stab them onto the bases as well.

Note that when I did it the first time I used some crappy foam board that was started to fall apart as I cut it, so I ended up wrapping the entire piece with clear tape. It added a bit of gloss to the finished product, but the colors got a bit dimmer.

By the way, Gundam rocks and I have fond memories of putting together my own mobile suit prototype wargames. Think I got a rule-set sitting around in some notebook in storage someplace. Best of luck!