I know this sounds crazy, but for 8 or 10 bit resolution, a stand-alone ADC costs more than a chip that includes both a CPU and a ADC.
(For the reasons Kortuk mentioned, 14 or more bits of precision are usually handled by an external dedicated ADC.)
The Atmel ATtiny13 is the lowest cost chip I know of with an ADC -- less than the MCP3208 or MCP3204 at my favorite distributor.
(I think you can program it to emulate a has 3 input 10 bit SPI ADC).
The Atmel ATtiny261 is the lowest cost per-analog-input chip I know of (it has 11 input 10 bit ADC).
If your analog sensor is far from your CPU, it makes sense to put an ADC right on the analog sensor and pipe noise-resistant digital samples back to the CPU.
Perhaps that "ADC" should be a second CPU emulating a slave SPI ADC.
(On the other hand, sometimes it's better to use a hard-wired chip that "just works" -- like the Microchip MCP3208 -- than to spend a bunch of time programming and debugging a microcontroller).