Is there one under $200? The higher the resolution the better.
If there aren't any (at all or within that price range), I will probably use a dedicated microcontroller to make my own serial port interface. Any recommendations on the most effective way to send data? For example, should it send data 20 times per second (start_byte, data1, data2, stop_byte), or should it be polled by the host device (a computer in my case)?