How much would a MINT copy of one of these sell for? Would it be more beneficial (money-wise) to grade a mint copy?
I have the opportunity to purchase a copy for $80. The seller states mint, but I would actually have to see when it arrived… 
At that price go for it! They’re selling at that price for played/damaged copies so I would do it in a heartbeat
There’s a lot of demand for mint error cards, and I would grade it personally. Ps. I sold a mint ungraded one for $300 last year. There have been few mint copies available lately.
Got it for $70. 
And they threw in a mint non-error Blastoise. I really hope they’re as mint as they say! I’ll post pictures when they arrive. 
$70 is a bargain! I once had the opportunity to purchase the card in PSA 10 condition for around $130 the guy who was selling it didn’t realise it was an error one because PSA graded it as a normal blastoise. Since I was new to collecting I passed it up which I now regret because as CBD1235 said they can sell for around $300 ungraded.