First of all, when buying RAM for your PC or Mac, make absolutely sure you’re buying the right specifications. Most RAM chips are not compatible with your Mac, even if they’re the capacity you want.
Second, any time you buy a particular brand’s certified OEM RAM – be it Apple, Dell, HP, whoever – you’ll always pay a premium. They do it because they can… but they back it up by guaranteeing your computer’s warranty support.
Third, you certainly can buy compatible RAM much cheaper from a third party. I just did exactly that, getting 8GB of RAM (in two 4GB DIMMs) for my wife’s MacBook Pro.
Fourth, if you do get RAM from a third-party, make sure you keep your old RAM on hand. If you do need to take your Mac in for warranty service, simply swapping the old RAM back in will put you back in compliance.
Fifth, your Mac is not “sealed”. You can swap out the RAM chips yourself. >> looking at you, @Tropical_Willie.
Sixth, when it comes to RAM for a Mac, I only look at Kensington brand. They’ve been making high-quality Apple components for years. As the old saw goes, you do get what you pay for.