I've been growing with artificial light since about '83 and here's how I do it and why.I buy the cheapest bulbs I can;period.but I change them every 6 months.why? because of the drop in intensity over time that occurs with all bulbs.you can spend tons of cash on bulbs and you'll be pleased with the beginning results.but the high dollar bulbs fail at the same rate as the cheap-o's.with the cheap-o's you can afford to replace them more often and then you keep your intensity higher more constantly.I also write the date I installed the newest bulb on the reflector so i can track its age.here's a place I use for cheap gear at great prices
Light bulbs, Rope Lights, Halogen and Fluorescent bulb - 1000-Bulbs - The Light Bulb Superstore