One difference (that still won’t justify the price difference) is that the Seamaster 300 uses LiquidMetal for the numbers in the bezel, while the 300m uses paint. The bezel markings on the 300 should theoretically last forever, while those on the 300m may fade over time and possible chip.
With that said, it really comes down to marketing. The Seamaster 300 was introduced when the older 300m was still in production and using the 2500 co-axial caliber (which had the co-axial escapement added to an older existing ETA-based movement). Omega priced the Seamaster 300 with its improved antimagnetic in-house movement at a significantly higher price (justifiably). When the new Seamaster 300m came out with its improved antimagnetic in-house movement in 2018, Omega decided not to increase the price significantly over the price of the older 300m. I forget the exact increase, but it was on the order of $500-750. Omega could have chosen to bump up the price even more, but I suspect they wanted to keep it lower to push up its popularity.
That seems to have worked.
