I want to make a PCB for this so everyone can have a TIMID soundchip. I want to know which features should be included on the board.

These features are optional and make the board larger and more expensive.
Onboard voltage regulator. (So you can power it using a 9v battery for example)
I/O pins broken out so it's a fullfledged arduino compatible board.

I can have the bare pcb's manufactured for about $1.20 each. The microcontroller costs about $2-3. The price of the other components varies, but I think I can get the total cost down to $5-8 (depends on the additional features of the board)

I would actually like the regulator because I don't have an external +5v source, but I do have a +9v for my arduino.

I'll buy if I can get my parents to let me.

Does it have to fit inside of a calculator or is it better to make it an external device with more features?

That depends on the amount of people who actually need one of these. The minimum order per board is 10 pieces.