(non volatile) RAM disk, looking for the cheapest storage chip

[legacy]

I am trying to add a storage device to my 68K board, I'd like to have something as big as possible
my board has 5 slots, each slot is capable to address up to 16 Mbyte over a 32bit parallel bus

currently in use  chips DS1265Y
chips, whose datasheet claims:

• fully static nonvolatile SRAMs organized as 1,048,576 words by 8 bits (1Mbyte)
• 5 years minimum data retention in the absence of external power
• Data is automatically protected during power loss
• Unlimited write cycles

the above PCB draft shows the initial idea where I planned to use four NVRAM, in the theory I have enough space on the each DIP socket to address 4 Mbyte per chip, but I can't practically find anything greater than 1Mbyte (I mean at cheap price, a NVRAM chip of 2Mbyte (1) costs too much), so the ram disk is going to be (1x4=) 4Mbyte of total storage size, just because I found 4 chips shipped for 50 euro (second hand, and they work fine).

OK, so here we are with the topic questions: what do you suggest, guys ? FeRam ?
Do you know a cheap FeRam chip, which comes with 3.3V/5V, 8bit data interface, 2..4Mbyte per chip ?
All of this, at cheap price ? (say, "it's cheap", if it costs a quarter than Dallas price for their NVRAM)

thanks 


(1) e.g. DS1270Y, 2Mbyte


edit:
ops, I had forget to underline: it MUST be non volatile, I mean able to retain information once switched off

[legacy]

it seems the biggest FeRAM chip is sized no more than 512Kbyte 

[miguelvp]

it seems the biggest FeRAM chip is sized no more than 512Kbyte 

Cypress has bigger ones than that:
http://www.cypress.com/products/nonvolatile-ram

[legacy]

Parallel F-RAM Features, Density: 64Kb - 4Mb
I have to check their packages, as I can't handle the BGA

[miguelvp]

Parallel F-RAM Features, Density: 64Kb - 4Mb
I have to check their packages, as I can't handle the BGA

I was thinking more the 16Mb one giving you 2MBytes
http://www.cypress.com/products/16mb-nvsram-replacement-battery-backed-sram-and-mram
Unless you can combine several 4Mb ones to give you more.

[C]

If this memory is not changing much, this might help.

If you had a memory management unit you could use the dirty bit to write a block to storage.
In place of a blind big memory block copy, you could do smaller what has changed copy.
Dirty bit = block of memory has been written to.

Dirty bit store is just an extra ram that does a read on every memory cycle and tacks on a modify write if WR active. This is where one bit in this memory = one block.
If the true power off device has a block size, you could make the dirty bit size match.

Not as simple, could be cheaper for larger memory size.

[legacy]

umm the CY14B108L seems interesting 

[miguelvp]

Would be 3V +- 20% be an issue?

Also not very cheap (just read the OP) but I don't know your definition of cheap
http://www.findchips.com/search/CY14B108L-ZS45XI

[miguelvp]

Cool just making sure you took the voltage into account and that $40 was cheap enough
Of course the one I linked was the cheapest one with 45ns access, the 25ns and 20ns ones cost more.