Thanks for the advice, I should have been more clear on the suitability part. It's acceptable as per the requirements. But what opinion that I would like to know is if it's good enough.

The design would require both a lot of I/O operations which the 8051 is good at, and math operations. Since it's not a time critical computation, large delays are acceptable(FX-991 take about a second for multiplying 2 3x3 matrices filled with large nos) so I felt that an ARM wasn't necessary. I could be wrong too, since we haven't actually gone about building it. As a very rough estimate:-), the code size should be moderate regardless of the uc used.

About the cost of Cortex M-3, no I haven't actually searched for the price, but aren't all ARM microcontrollers relatively more expensive compared to 51s and pics? Over here and AT 89c52 costs about RS 40. The 991ms cost about RS 550 and an es costs about 700. Over here I would require an 8052, an LCD, keypad, RAM and probably EEPROM. 40,110,x,75,54(serial 64K EEPROM). That comes to RS 279 without considering the board, and keypad. The external ROM may not be necessary.