If you are looking at a battery for a RX pack, check ebay. There are a few cheap $10 packs that fit the 5-cell 2/3A recess.
If you want a cheap battery for the B4, use a yeah racing pack. For $35 my pack has never let me down.
If you need a custom battery, then there are no rules as long as it suits your needs.
Before SMC released thier 3.7V lipo, I made my own for testing.
I'm using the BatterySpace 2100mah cells in my 1:12 lipo car. I wired three 2100mah cells in a 3P configuration to make a 3.7V 6300mah battery. In the pan car, they feel zippy with a 13.5 Motor. In my B4 buggy and a 7.5 Motor, the buggy looks like it has a old mabuchi motor in it. I guess the motor needs some extra volts for the tranny to rev up. I was able to see brief loads up to 60A with the 7.5motor, so I know the batteries are working. I guess, I can always bump up the gearing or drop in a faster motor for stress testing.
"If everything seems under control, you're just not going fast enough." - Mario Andretti