well here in this forum you can fine one and they will cost from $500 up to $1000 USD and am thinking of selling my but its not finish if i end up selling it al contact you
Not really. GameCube Portables usually sell around $500-$1000 as thecasperrojas said. You might get lucky though. But $300-$400 is just too low. That price range could get you an N64 portable, but not a GCP. What people do not realize is the time it takes to actually make these things. Hours spent is usually like 60-100 hours easily. Plus part cost along with labor.
I have an unfinished one I could sell you for around your budget. I'll be honest, I'm not the best case maker and some things need to replaced/fixed. Nothing too major though. PM me if your still looking.