This would be far easier if you were more specific. How many units do you plan to run that's going to massively affect your per unit cost. The major parts of an uno, a motor control IC, and 256mb of flash on a custom PCB can easily be under $20 a unit with decent volume really anything not using a very expensive IC can come in under this ammount. Do you need USB accessibility, I assume so as you say downloadable or do you need a sd card that's removable? Do you need a 7.2v battery if so why normally better to figure out watt hours and/or sizing/weight/chemistry requirements. 3A motor means little how many volts? Wall charger do you really need this if your programming via USB why not charge via it?