What would you consider NECESSARY for a FOWLR that would add up to that much? By my math, for an average 4ft being on the high side for prices..
4ft tank Dunno, say $1000 inc stand.
Base rock: $300
Skimmer: $500
A couple flouro fittings $100
RO $200?(Haven't looked at prices)
Heater: $30
2-3 Power heads: $100
Test kits $100
Refractometer $120
Then add whatever livestock on top of that. So say about $2500 and that's with a skimmer that would be better than you need for just fish. You can get the tank for heaps cheaper than $1000 and odds are you'd already have one. So that's heaps less, same can be said for the heater.
Now, if you want to make wavemakers, temp controllers, calcium reactors, peristaltic pumps as others here would suggest, just for your fish...Then yeah, it can cost that much...
Let's see, the equipment I have that would be necessary for just a FOWLR would be about $1300, most of that being a POS Deltec skimmer and rock.