Clamp-on DC current meters are also readily available off the shelf. Finding one that can easily be interfaced is harder, didn't find any in a few minutes of search. Can't imagine they're not out there.
Now for "how much it costs to code" you have to provide a detailed specification of what you want to do with that signal. You want it displayed I suppose - on what type of display? A 1602 LCD is dead easy.
I assume you have the clamp-on part working, and get a nice signal out of it from an analog input, and that you have the calibration curve to relate those numbers into actual currents, so the code can basically take the actual current value as input.
The most basic code is something like:
void loop() {
float ampReading = analogRead(ampReaderPin) * calibrationFactor;
lcd.setCursor(0, 0);
lcd.print(ampReading);
lcd.print(F(" A "));
delay(500);
}
This shows the current current every 0.5 seconds, that interval is of course trivial to change. Setting up the lcd and so I left out as it depends on how it's connected (I2C or parallel) and the library used, but it's still quite trivial.
Any buttons for control - e.g. hold the value, range, etc? Now it gets a bit more interesting.
Do you need averaging over time? If so, how many samples over what time period?
Other functions you need?