2 different kinds of ballast.
Magnetic.
Digital.
Magnetic are cheaper and you can buy 250w, 400w, 600w + 1000w.
Digital ballasts are usually switchable meaning if you buy a 1000w ballast you can switch it to 400w or 600w as well as have it running at full capacity 1000w.
if you have a 600w bulb, you need a 600w ballast. and so on.
Depending on the kind of plant you're growing (auto/photo) should decide which type(s) of bulb you need. The 4 major types of bulb are Flourescent (and compact flourescent), LED, Metal Halide and High Pressure Sodium. The latter 2 are classed as HID (high intensity discharge) lights, which you need a ballast for.
They all depend on your preference. Some have different lumens per watt and different colours like blue or red light. ideally for veg you need more blue light and flowering you will want more red/orange light. The colour is measured in K. the higher the K the more blue/white light and the lower the k the more red/yellow light.
They all cost different amounts to buy and run and some produce more heat than others. You should take that into consideration relative to where you live. If it's really hot where you are, you probably won't want a huge 1000W hps light kicking out even more heat that you have to cool to stop the plant suffering, adding more cost to your grow. So you'd be sensible looking into cfl's or led lighting.
If you're in a cold climate, a light which produces heat which will need little cooling to keep the plant comfortable is probably a good option. Paying a little more for optimum conditions to your situation will increase your yield over the long run and end up being more cost effective.
It's all a matter of finding what you need to best suit your needs. Without a lot more information I doubt many people will be able to help you. They know relatively nothing about your grow space/room. There is no "this is the best for $100" but you would probably be advised to go for a 400w hps on 18/6 straight through for your auto lowryder.
Do a little more research on lighting then you will know yourself

Hope this helped you.
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