When I say "really good" 5 series i'm referring to top of the line 5 series, 5850 is middle of the road as there is a 5970 which would blow it out of the water. From an economical perspective, yes crossfire/SLI will always be cheaper than buying a single high-end card, but the performance will be lower as well.
Again I'm not too familiar with ATI/AMD's video card lineup, but that's how it works in the nvidia world. Also with nvidia, a higher number does not imply better performance, it simply refers to newer generation. For example an 8800 card could be faster than a 9600.
You get what you pay for is true, but the cost/performance ratio does go down as you get into the higher end cards (you pay exponentially more for linearly more performance). For the people who just jump on the brand new, absolute best graphics cards are always making a poor economical decision. SLI/crossfire can be a feasible option for people who want more performance for a cheaper price, it all depends on the models in question. It looks like you've made the proper research involved with the models you're looking at it, so you got a pretty good deal.
