Well, for a few examples here:
Retail prices in the US:
8 Chaos Space Marines = $25
5 Grey Knights = $30
20 Cadian shock troops = $35
10 Sisters of Battle = $35
Basic troops choice costs, no upgrades or sgts:
8 Chaos Space Marines = 112pts
5 Grey Knights = 125
20 Cadian Shock Troops = 120
10 Sisters of Battle = 110
Once you add in all the different upgrade options for each army you'll find they can all cost about the same. All you'd really be saving on is painting time. You still spend about the same cash for about the same points with any army you choose. Armies with bundle packages like Transport+Troop, Battleforce, or Megaforce, are the ones that can be cheaper by having those extra deals available.
Other then that, it's whatever you find on ebay, or a store's clearance shelves.
So my adivce remains:
Pick an army you like the look of the models.
Buy the codex, read through, and pick out a starting army of units.
Look for those units on ebay, or in bundle-deals, or both.