This extension is only possible due to very modern part (2017?), a $25 AS6C6416, 8MB SRAM in a TSOP Package https://www.alliancememory.com/wp-content/uploads/pdf/Allian... the rest is really cheap and very simple. You could replicate it at $50 bom including shipping for all the parts. Thru hole would indeed push the price to >$150.

Even still, I am a bit dubious about this Ram chip. Its 3.3V rated, and its maximum ratings are below what Amiga presents on its bus (5V). I wonder if the fact its working is the miracle of modern manufacturing processes and safety margins, or just a temporary fluke and will results in dead parts in near future. Normally you would need either a 5V tolerant part or a bus translator like <$1 74ALVC164245.