Compare? Not sure. I can tell you Vent on typefrag is great tho, and you can get a trial offer for a server. After that it's not expensive.
If one person can't pay for a server, have everyone who raids chip in and send you or the GM a buck or two via paypal every month.
Raiding without voice comm seems like a nightmare to me. Maybe some day Blizz will fix their in-game voice chat, but until then you'll have to use a third-party server.
Edit: on the other hand, maybe you're trying too hard to make your guild a raiding guild. If that's not their culture, perhaps finding an established raiding guild (with a vent server) is easier than bending this one into what you want. Just a thought.