You'll just need to spend a lot on parts
Some creative genius made his own Metal Gear Rex out of freaking LEGO. It's huge, weighs six pounds, features numerous working parts, and did I mention it was huge? Screw buying ThreeA's official Metal Gear Rex figure for $500! You can just make your own after spending $900 on parts ... oh wait.
Still, if you're willing to spend that much then check out the instructions and see all the parts you need in order to build your own awesome version of Rex.
LEGO Metal Gear Solid REX V.2 modeled by ragnarock01 [Gunjap, via Reddit]