I've added 3 different types of ammo.
Light Energy Cell (damages mobs)
Medium Energy Cell (damages mobs and blocks)
Heavy Energy Cell (explosive)
Light Energy Cells are crafted with 1 bit of plastic and 1 bit of redstone - by default it'll give 3 cells per craft, but this will be configurable to allow people to balance better based on what mods they have installed.
Medium Energy Cells are crafted with a light energy cell, some plastic and some glowstone
Heavy Energy Cells are crafted with a medium energy cell, some plastic and some gunpowder.
Of course, you also need suitable upgrades installed in the robot to enable the different weapons.
For a tier 1 upgrade to enable fireLight() you need plastic, an optical lense and a microcontroller. A microcontroller is created using a comparator, carbon, and wires. carbon is created by smelting coal, wires are made from iron and redstone. An optical lense is made from plastic and thin glass.
For a tier 2 upgrade to enable fireMedium() you need a tier 1 upgrade crafted against a focus lense. A focus lense is made using two optical lenses, plastic, a microcontroller and a prepared circuit board. A prepared circuit board is crafted using PCB together with a resistor, transistor, led, capacitor, optoisolator, wires and a microcontroller. I won't go into how to craft each of those!!
For a tier 3 upgrade to enable fireHeavy() you need a tier 2 upgrade crafted against a lazer sight. A lazer sight requires a solar cell, plastic, led, ribbon cable and a prepared circuit board.
... yeah, these robots are going to have a pretty complex crafting tree. However, I think it's a good thing. The robots are there to be used as end-game enjoyment of programming, not as tools to make life easier. I want people to feel like they've achieved something when they get their first robot..
Of course, all the other upgrades have a similarly complicated crafting tree, and the robots themselves are also very complex
Oh, also - the recipes aren't really expensive at all. Theres no rare ores or anything like that you need.