Check
https://gastracker.uk (there are other sites but this one is my fave as it's easy on mobile and the interface isn't cluttered), and choose to see over the last 365 (or longer) historical rates. It'll show the average price of tracker over that period and you could calculate what you'd have saved had you been on it.
Since Jan last year, I've found the price is always around 1/3rd cheaper and that's reflected in our monthly bill (from £180 down to £120). There are times when it's spiked really high but for a few days (and only on electricity I think) but the price for the majority of the days have been below the price cap.
In fact if you turn on the SVT button (the price cap rate) you'll see the tracker rate is mostly under the SVT line, and the gap or the area between them is the actual savings per unit of energy.