Spend the $30 on DDR3.

Yes. That’s a no-brainer.

A full platform upgrade—CPU, motherboard, RAM—will run you at least $200–$300 even on a budget. For a 2015 system, $30 for 16GB of DDR3 is the cheapest performance boost you can get. It’ll make multitasking smoother, keep the machine usable for another year or two, and doesn’t lock you into anything.

Saving that $30 won’t get you meaningfully closer to a new platform. You’d be waiting months for a couple hundred bucks while your current system chokes on 8GB. By then the hardware you’d want might have changed price or been discontinued anyway.

If the rest of the system still does what you need (decent CPU, SSD, etc.), drop the $30 in and call it done. Future You can worry about the upgrade when this machine actually dies.