Yep, $280 is the cheapest buy-in I know of, $318 for the one I bought. Even at that, I consider it a good investment. The amount of insight you will gain, and ability to get exact readings from bulbs has easily saved me that money. After having used one for a while now, I consider it just as important of a tool for the reef as a hydrometer. You need to keep track of the salinity in the reef, and be exact, no? If your salinity goes outside the 1.020-1.026 norm, you could have serious problems. Well, knowing the amount of photons in the water is just as important really.
It allows me to set up my bulbs/reflectors ideally, and know when its REALLY time to change my bulbs (not just a guess like most reefers). When changing bulbs, I can use the meter to acclimate the tank to the new bulb's output by raising the bulb/using mesh, etc and matching the true outputs. When I buy corals, clams, etc... I can take a reading from the tank they came from, right where that coral came from, and match it in my tank. I take 'mapped' charts of other's tanks, like this...
And discover that even though the owner of the above tank has placed the highest-light corals at the top of the tank, that they are getting less light than the ones at mid-level because the reflectors arent distributing as much light to the top back of the tank as the middle. Ah ha! Thats why they arent growing/looking good! Time to adjust the reflectors!
I can tailor lighting to be just the right amount for a tank with proper reflector use and placement. I can make sure that the top of the tank gets 400-500 readings, the middle gets 250-350, and the bottom gets 100-200... and be SURE of it. If my corals start to not look so good, I know if lighting is the reason or not.
You would be shocked how so many seemingly unimportant things can change the light output or distribution. One coral only inches from another might get a 100-200 micromol/m^2*s drop in intensity that wouldnt be visible otherwise.