I've been happily using the Camelizer for years: https://camelcamelcamel.com/

You can visit the site and/or install a plugin.

Been using the Camelizer for years! It's a great tool.

I would love if one of the extensions mentioned in sister replies would include a feature that hides the "regular price" so that you can't be falsely anchored (and duped into thinking you're getting a good deal).

All you'd see is the price you're being asked to pay, so you can evaluate whether it's worth that price or not.

I've been using the Honey extension on Amazon for the past few months. Averaged out, it has saved me a little under 10% of what I would have spent, so I am happy with it. At least this way some of that marketing money is coming back to me. It has also incidentally shown me just how often Prime items are jacked in price.

Honey made Firefox extremely slow when I tried it out a few months ago. I was using PriceBlink but their latest update also slowed down Firefox considerably.

I can report that it is well-behaved in Chrome, at least.