I doubt if any calculator chip, if available, would interface with anything but its own custom-built LCD display and its own membrane keypad. That does not lend itself well to prototype work.

If you want to make a calculator, then you want to program it yourself, right? Otherwise there is no point. What would you learn by assembling an off-the-shelf calc? If you want experience doing that, take a heat gun and remove all the components from a calculator and then solder them on again.

You can't get much cheaper than an 8051-based prototype setup:

Crystal: $1
P89C51RD2: $6
Max232: $1
Caps: $1
DB9 conn: $1
LCD: $10

Then find a suitable keypad and interface it.

Of course I am assuming that you have a power supply, protoboard, computer, and an oscilloscope. Hard to do work without those.

- Lee