The question is whether they are "new" or "old" style raiders. At that price you may have gotten the older ones.... which back in the day, GW did sell a ravager upgrade blister for. I have watched these off and on and they sell well over $20. That said, they are metal, and people did all kinds of neat modular tricks with them. Still I'd never pay $20 to upgrade something that cost me $10 to begin with.
The newer Ravager is just a raider with an upgrade sprue which includes the extra gunners/gun options. I have seen the upgrade sprues up for auction but they are tremendously expensive for what you get. I'd say go for the individual bits; you can pick what you absolutely need and it should be cheaper. If you want to try your hand at creating your own however - I'm sure there are lots of cheap dark lances floating around from scourge/warrior box sets.
Either way - Good Luck
EDIT: Image of the Ravager upgrade sprue for reference