Aquatraders have cheaper $8+shipping replacement bulb for the Coralife x3 UV's
http://www.aquatraders.com/category-s/8.htm
They have x3, x6 and x12
In terms of UV usage, do not over use it if your system is new (less then 1-2 years old). My present system is 6 years old and i only use it when bubble algae is growing too much and or when fish is showing scratches from getting too close to corals to prevent infection.
I have mine plumbed together with small bio pellet retrofitted
(TLF) reactor and water flowing through at all times, but i only use UV for a week or too to prevent fish infection or to get rid of free floating single cell algae. Then i turn it off.
Also a good note on UV that is you can easily control how it will sterilize by controlling the amount of water flowing through the unit.
Less flow more sterilizing, more flow less sterilizing. The point is that you do not need huge UV light you can get good results from smaller unit by controlling the flow.
HTH.