HUGE newbie question I know. First time grower, so be take it easy guys. 
So I am ready to order a ballast and a bulb. Specifically from here:
Ballast: 400 Watt - High Pressure Sodium Ballast - Multi-Tap - Sola #E-971-WDD-SO400 Light Bulb
Bulb: 400 Watt HPS
I should be good to go right? (also, why is one bulb 50+ dollars)
Unless I am a bigger idiot than I thought, I'm ready to go.

So I am ready to order a ballast and a bulb. Specifically from here:
Ballast: 400 Watt - High Pressure Sodium Ballast - Multi-Tap - Sola #E-971-WDD-SO400 Light Bulb
Bulb: 400 Watt HPS
I should be good to go right? (also, why is one bulb 50+ dollars)
Unless I am a bigger idiot than I thought, I'm ready to go.