It wouldn't be anything like the N64 days where a cartridge cost $30/unit for third parties to manufacture.
Most likely what will happen is most developers will use a very cheap card, like 8GB or 16GB, enough so you can play the first 3-4 hours of a game, but the rest of the game you'll have to download at home, so you'll need an internet connection.
Though this wouldn't really even be rare for disc based games, every damn game today it seems like once you get it home requires some kind of patch/download/etc.
In that sense it could be something like $1-$1.50/card for third parties when you factor in mass production, I mean sure more than a disc, but not enough for a developer to give that much of a crap. If the N64 had $1-$2 cartridges with GBs of data, it would've sold at least doube the amount because it would've had tons of third party support.
