Define 'low cost'.
What other requirements are here? Are you studying to actually use microcontrollers, because I've been burned by this before. If you are studying for a profession where using a microcontroller is helpful or advanced use is required, your answer is different than if you just need to get this one simple project done and you're actually an artist or something.
If you just need to get this done and don't actually care about learning about microcontrollers than I would vote a cheaper Arduino spinoffs.
If you want to learn something that will be used later I would use a microcontroller that has a high amount of hobby support. At this time that would be PIC or AVR, or possibly MSP430 since they have hit the hobby market hard. This is a very simple task your asking and finding a microcontroller with dual UARTs is not that difficult.