Cheapest ARM with an MMU
The Atmel AT91SAM9260B comes in a LQFP package and may be a little bit lower in price than the SAM9G20.  It has a MMU and the same peripherals as the SAM9G20 but runs at 200 MHz rather than a max of 400 MHz.  Pay attention to the details in the Atmel Application note Schematic Checklists and you will be successful.

You could also try the AT91SAM9XE512 with flash on-chip.  But 512KB isn't going to be enough even for uCLinux, so the most cost effective thing to do is use the flash-less SAM9G20 or SAM9260B with external flash and SDRAM.

http://www.atmel.com/at91