The problem is that to fit on the existing board, you need a single RAM package with the same number and function of pads. Looking at a sample photo of a board, I can see it has a Micron package labelled D8CJN. Looking that up on Micron's FBGA parts decoder, the full product code is MT53E2G32D4DE-046 WT:C which is a 64Gb (64 gigabit, i.e. 8 gigabyte) LPDDR4x RAM at 2133 MHz, in a 200-ball TFBGA package.

Looking at Micron's range, they now have a 128Gb (16GB) chip with matching specs, which is the MT53E4G32D8CY-046 WT:C. That chip costs £74.66 per chip from Mouser Electronics (just one site I found selling it) in quantities of 1,360.

In contrast the 2GB chip - which I surmise is a MT53E512M32D1ZW-046 - costs £9.26 each in quantities of 250. There's a price break listed for 2,720 of the things, but I'm not going to ask for a quote.

So that's £65.40 more for the larger RAM chip. Which is about $80 on today's mid-market exchange rates.

Looking at the 4GB it's probably a MT53E1G32D2FW-046, which is £16.72 in bulk quantities of 1,360 from the same wholesaler (for the revision C part). So that's an extra £7.46 or about $9.

Why are other systems' RAM upgrades cheaper? Largely because they use more chips, and therefore each chip has less capacity. That means either the chip is less dense, or it's a smaller die, so fewer defects are likely within the one chip, making yields higher. An SO-DIMM might have two or four chips on it, depending on whether it's populated on one side or both; a full-size DIMM might be 8 or 16 (or 9 or 18 if it has ECC).

Obviously I've made the assumption that this is just a drop-in replacement chip on the existing board. But I assume it would be very hard to redesign the board to fit in the same form factor, with all connectors in the same positions, but add on another RAM chip. If the SoC even supports driving more than one RAM chip, which it may not.

TL;DR that's what Micron charge for the larger chip.