I found this interesting resource that appears to list the introductory pricing when devices appeared. Some items are listed as DRAM (I suspect that this is all forms of RAM).
This is not a complete answer but it is way too big for a comment.
One thing to keep in mind (particularly with semiconductors at the time as consumer electronics was becoming popular) is that the price falls over time as more are produced and processes upgraded, so even though the introductory pricing of 256Kbit (32 KB) is shown as $15,000.00 per MB, that would slowly (perhaps not so slowly) drop over time until end of life (when the price rises dramatically).
Note that the price above is for a device introduced in 1978; a 256kbit part that was then introduced in 1979 (4 months later) was priced at introduction at $10,528.00 per MB, a reduction of about 30% in just a few months.
Given that, I would expect the pricing of 256K parts to continue to drop (and in line with newer parts) until the manufacturers simply were not selling enough to make them economically viable.
That would take us to the time when older processors were dropping out of fashion and newer processors needed significantly more memory and even then (perhaps more so then) PCB space is at a premium so if you could save space (fewer parts, smaller PCB) it might make sense to pay a little more for one IC rather than 2.
The older processors (certainly the perennially mentioned 6502 / 65C02) were still in production until at least the mid 90s and would therefore have supported a supply chain producing 256kbit parts as early prices for the denser parts was still more expensive on a per bit basis.
I don't have any data for 1Mbit parts, but the 2Mbit (256KB) from 1982 (made by California Micro Devices) was initially priced at $1,980.00 per MB and as previously mentioned, I would expect that the cost per bit over the life cycle would continue to drop in line with the newer parts.
By the mid 1990s, the pricing for new parts was at £32.00 per MB and I would expect mature parts to be somewhere around that and perhaps up to twice that for the older (256kbit) parts of perhaps $64.00 per MB as the prices would have been rising over the past few years.
So the continuance of the 256kbit parts until the mid 90s is really not surprising and I would expect pricing to start increasing for the lower density parts around the early 90s (fewer produced, freeing up fab space for newer parts as demand for those increased while the demand for the lower density parts decreased).
Manufacturers used a number of tricks to keep (or push) the prices down; one such was done with the 6116 SRAM. If a bit (or more) in one logical half of the device was faulty, it was sold as a 6108A or a 6108B (depending on which half was good). This was possible as they were both in a 24 pin package and the only difference was the implementation of A10 (for the 6116).
In at least one console (Atari IIRC) there was a jumper next to each device that would select either the upper half of the part or the lower half (by setting A10 for the device either high or low - the position was irrelevant for a 6108).