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Cheapest with enough resources is ATMEGA48 (QFN32 ultrasmall package). Also excellent debugging support via RESET pin (when using JTAGICE).
Actually you can go over this speed 115200 by using oscillator 14.7456 MHz - it has the magical quality to generate all possible baud rates with ZERO ERROR! I made many projects with 230400 bps and even higher. There is no limitations to go to almost 1Mbps
all the microcontroller supports the baudrate
only thing is that you have to check the equation which makes the baudrate and the frequency for the same.check whether the purticular MCU is supporting the frequency