Brand new out of the box cheapest army is the chaos Space Marine Megaforce. If I were starting out today and wanted to play right away, this is what I'd go with.
If you go ebay. are patient, and aren't picky about which army you play; then you can get some good deals. Last month I got about 2000 points worth of models for $96 and some changed (shipped to my door).
My favs are in this order:
World Eaters (Khorne!)
Khorne lead "Black Legion" (Khorne & Chaos)
Nurgle lead (and themed) Lost and the Damned
Chaos Undivided Black Legion (Undivided - sometimes backed by Khorne and Nurgle)
If you go ebay. are patient, and aren't picky about which army you play; then you can get some good deals. Last month I got about 2000 points worth of models for $96 and some changed (shipped to my door).
My favs are in this order:
World Eaters (Khorne!)
Khorne lead "Black Legion" (Khorne & Chaos)
Nurgle lead (and themed) Lost and the Damned
Chaos Undivided Black Legion (Undivided - sometimes backed by Khorne and Nurgle)