Let me think on the biofilters... I built two 55's. $40 for food-grade barrels, around $60 for 9000 feet of plastic strapping, $20 for the plastic light-fixture gratings, $6 for mesh laundry bags, $20 for green scrubbie pads, and somewhere around $40 for the 2" pipe plumbing fixtures (swirler in the bottom plus drains, exit spouts, and pipe). So that all comes out to $186 plus the time to put it together -- $93 for each filter.
So yeah... around $100 sounds just about right. That's assuming, of course, that you have the tools on hand such as hole-saws to mount the pipes into your barrel.
I think the hardest part for a newcomer, when trying to design their first waterfall, is that you really have no visual concept of what XXX gallons per hour looks like. We can't even use a garden hose as a measurement, because every city's water pressure is different. It sounds like your local store is going to help with that by having a waterfall display, and the good thing is that if your water flow is too low, you can always add a second pump that ONLY flows over the falls.