While blister by blister, they're the same cost, the points costs are very different. Undead or Skaven for example will require roughly double the spend as High Elves or Empire, and Chaos is so points heavy that you only really need a dozen or so units for 2k points.
I hear you on the price thing, I'm currently in the process of buying an araby army bit by bit, and it's going to cost me a fortune. Araby is a latecomer to Warmaster, and though they're not OOP like Kislev, there aren't any deals around really. If you're interested in Empire, you might want to check out Azrael's bundle. A bud of mine got a high elf bundle off him, and even with shipping to Canada is was a substantial savings over buying new.
As for going with other miniatures, I hear you, I only use the GW ones myself, they're by far the best 10mm fantasy miniatures out there. It takes me so long to paint that it's pointless not to pay the premium on the best minis to have the best possible looking force on the table.
Great game, I strongly recommend you get into it.