I'm late to this party, but I would like to point out something that often gets neglected in these types of cost/performance analyses, and that is the "human time" factor. It would take quite a bit of additional cost in hardware before I would consider implementing a solution that involves double the amount of physical computers for the same performance. Over the lifespan of this setup, how many hours do you think will be lost to configuring, maintaining and trouble-shooting these machines? Monitoring Windows updates alone would drive me insane... Anyway, cost/performance is tricky because it often fails to factor in those activities outside of processing and power consumption. There's also the additional equipment to deal with (cabling, power supplies, switches, hard drives, etc.) that can drive up the cost over the long run more than is evident at the start...
Good luck!