All of our GameCube guides are located in the guide hub. However, the cost of a portable is very expensive, and as Wiis are only $15-20 it is absolutely not worth it to build a GameCube portable anymore. GCPs get less battery life, run hotter, are much harder to build, much more expensive to build, can't run game backups without super rare modchips, and are generally less reliable than Wiis.
The previous message was sponsored by ShankMods: Living in a :NOGC: world since 2016.
As far as a final cost for a goes,first portable I'd say you should expect to spend $100 at least on tools and materials, and the actual build will likely cost over $200 in parts. Portables aren't cheap.