I have something comparable to this: H11 55W 3158lm 5000K Car HID Xenon Lamps w/ Ballasts Kit (Pair) - Free Shipping - DealExtreme
That is lamps, cables and ballasts together for less than $25. An extra pair of lamps would set you back for about $8.
Only problem is that this set does not have a proper H11 plug; you have to shove the metal pins in the connector, then tape it up thoroughly.
The rubber plug is for small lamp housings; the ballast would not fit in them so you'd have to drill a hole, feed the cables through it to the ballast outside and the lamp and connectors inside.
Our insight does not have a lamp house so you can cut away the rubber plug to free the cables and mount the ballast wherever it is out of the way.
I'm happy with my set (at least, the second set of lamps that now has about 2000 hours of use on them).
You can make your own choices, but I doubt you'd ever regret getting HIDs.