How to do math operations in android shell?
BusyBox doesn't include bc, but it does include the Reverse Polish Notation calculator dc.  Since Busybox can be installed from several installers on Google Play, this is probably the easiest way to get a command-line-based calculator on Android.

Alternatively, I've heard that bc can be compiled for Android quite easily using the Android NDK, so if you're comfortable with compiling software you could do this.