You can research prices on eBay. Look at sold listings first since it shows what people have actually paid for the item in the past. Then look at active listings to see what the current asking prices are. Condition is everything here so you'll need to look at each listing to compare yours.
If your copy is indeed pristine (no rips in the seal, no box damage), it looks like you can expect a sales price of around $1,000 for an ungraded North American sealed Super Mario 64 if you were to sell it on eBay. Of course, eBay will take 12% of that, plus you'll need to pay for the shipping (probably around $10-$25 for the label and whatever you need to spend on supplies). Also you need to ship with signature confirmation on any order over $750, so that's about $4 (if you don't ship with signature confirmation in an instance where it's required and the buyer says the item was not received, even if tracking says it was delivered eBay will still force you to refund them so you'll lose the money and the item). I would also probably recommend buying insurance on any package this valuable, which according to USPS will run you another $32 (while USPS will pretty much automatically cover a claim for a lost, undelivered package, it's much harder to be reimbursed for shipping damage - you'll need to follow USPS's requirements which include double-boxing and spacing requirements and have proof that you packed it well enough. And there are no guarantees that they would reimburse you even with all of this proof).
So from this $1,000 sale on eBay, you would get somewhere between $800-$830, pre-tax, assuming nothing goes wrong. If you're not experienced in selling online (which seems to be the case), I'd probably recommended just taking it to a game store, where they'll likely offer you around half of whatever they think they can sell it for - in this case, you'd probably be looking at walking away with $500-$600. A little less than what you would get selling it yourself, but you eliminate all of the potential risk.