are you talking PC's? with reef tanks the watt per gallon rule really doesn't work, it is the amount of PAR that the corals get
PAR
The expression Photosynthetically Active Radiation, often abbreviated PAR (sometimes shown as Photosynthetic Photon Flux (PPF)), designates the spectral range of solar light from 400 to 700 nanometers that is useful in the process of photosynthesis
Most T-5's last 18 months if they are not overdriven, PC's last about 8-12 months and MH bulbs can last from 8-12 months but change color after burning a long time.
$10 sound kinda cheap haha 6500k is a very yellow bulb and is used for growth mostly...your gonna want some actinic (blue) in there and I am sure those bulbs are more money
HTH