$400 is a good going rate for a decent skimmer for a 100g tank. Better than that $400 is great for one of the best skimmers ever.

You'll be better off buying 3 of the cheapest 5g buckets of salt for $100 than any skimmer for $100 as you could do 600g worth of water changes. At 30g a week you could keep your tank looking nice for several months while you save up for areal skimmer, plus you'll quickly realize why a good skimmer is so important. You'll only need to do a good water change once a month instead of once a week.

I don't even think you'd find a "quality" used skimmer for $100 to put on a 100g.