The expense was probably quite considerable. Not only do you have to have the game ROM on a chip, you would also need Nintendo's lockout chip too. If your game had a battery save system (DKC did) you would also need to buy a RAM chip and watch battery too. That's ignoring any enhancement chips as DKC didn't use any (but many other late generation games did).
And all that before you get to the fact that the only who could officially make these boards was Nintendo. Meaning there isn't exactly much competition driving prices down. Sure, Nintendo couldn't quite take the piss the way they could in the NES days, as Sega was all too eager to try and attract new games for its console, but unless you wanted to completely remake your game, you're dealing with the big N's bullshit.
The boards could probably have been made much cheaper today than in the 90s, as ROM memory was expensive AF, even the couple-of-MB ones used in the consoles of the day.
There's a reason PS1 and Saturn games were massively cheaper to buy than N64 games.
While we're at it, let's remember that this version allows for portable play that can also be plugged into a TV with nothing more than a cable and Bluetooth controller. Or if you really want to play on a PC setup and have a Mac (hint: if you're in the market for an iPhone 15 Pro, it's likely that you do) you can switch to that at no extra cost.
You may not pay an extra $30 for that, but plenty of people would consider that reasonable.
Are you talking about RE4? Because they were actually talking about an Apple port (iPhone, iPad and Mac, with people being able to play on all platforms with one purchase) of the recent remake, which is a 2023 game that only really borrows the story and some layouts from the 2005 game.
They're targeting known hardware. They can optimise right down to the specific card in use. They don't need to consider hardware that doesn't support these optimisations.