I think that one of the reasons that Moo's prices are higher is that you can choose many different images for the front of the card and still pay the same price. Another reason is that they us 16pt stock, whereas most other companies use a 14pt stock.
But yes, there are definitely may cheaper options–Moo is clearly marketing itself as a premium product, especially with its odd-sized business cards, etc. I looked at sharpdots.com and I like that they will do a hard proof for you–that could very be useful for doing a large offset run if you need really good accuracy. Usually for large runs that I do I don't need any kind of accuracy, and in that case you can get cards for really, really cheap. Actually it looks like sharpdots is quite competitive in offset... that's really good! Thanks for the tip, Chris!