I guess it depends on the tank and how closely you follow the RC algae scrubber basics thread.
According to "Floyd R Turbo" for example, there are certain requirements.
1) 35gph of flow, per linear inch of screen.
So, for my tank, assuming some head loss and that I want a reliable pump, there is some head loss and I'd need about a mag 7. So, that is 70W and would cost about $6.13 per month to run.
2) 0.5 to 1W per gallon lighting, actual, not "equivalent wattage" run for 18hrs per day.
So, for my tank, that would be $4.6 to $9.2 per month in electricity.
Then the bulbs "need" to be replaced every 3 months. In bulk, CFL bulbs seem to be about $0.25 per watt. So,
that would be $70-$140 per year.
So, following "Floyd R Turbo's" recommendation, the operating cost would be something in $199-$324 per year range for my tank. I only paid $370 for my XP2000 skimmer, and using the same method for electrical calculation, the operating cost is about $26.28 per year. So, just the operating costs would likely catch up to my skimmer in under two years. That doesn't include up front cost. A mag 7 is about $70ish I would want something safe, so, the bulbs would need acrylic splash gaurds etc... so, the initial cost, would be somewhere around $100. I seem to keep skimmers for several years. I guess if you replace them more often, that is another story.
I could buy used stuff, but I could also buy a used skimmer... I could skimp on changing lights and such maybe you think you don't need that much. Floyds method is trendy now though, so, that is what I am going by...