Welcome to the 405th.
If you're asking about the price, foam is cheaper way. Resin, fiberglass and bondo are definitely more expensive, but you're not spending 100$ (or more) at once.
Making armor will take you few months, so you may be spending 10-25$ monthy, more or less. With both resin and fiberglass.
While you may need around 10l resin, some body filler, fiberglass and paper. But if you go with foam, you'll need more than few big foam sheets, glue and more.
I don't think you should decide on what's cheaper. More like - what do you prefer. What do you think would suit you to work with.
You can try both on small pieces to see what you prefer.
I myself, I thought first I'd craft with foam, when I was testing it, I didn't enjoy it. Then I tried pepakura and I loved it, so I chose this way.
If you want Halo 2, so basically you want Mark VI as it's the same armor in both games.
Just depends on how detailed you want your armor to be, there are different versions of it.