Actually $9 more to go from 2GB to 4GB, just for the RAM chip itself. See my reply above.

32GB is impossible at present. No-one makes a 256 gigabit (32x8) chip to fit in the same footprint, so then you have to do a new board design to support multiple chips, which might not be doable in the board's current footprint.

Micron do a 192 gigabit (=24GB) chip but it's LPDDR5, rather than LPDDR4x, which I imagine the RPi 5's SoC can't drive.