My home PC's video card died (not directly related to mining) and so I just purchased a 5850 on newegg a few days ago. $185 - $30 rebate.
Assume 300MHash/sec:
- $185 (assuming I don't get the rebate....) = ~$0.61667/(MHash/s)
- $155 = ~$0.51667/(MHash/s)
So, if you already have "overhead hardware" to put the cards into, it can still be profitable. But, you are absolutely right that specifically building a rig is really pushing that line to where it is more profitable to buy BTC instead of mining hardware.
Edit: Also, I needed to purchase a video card anyways just for general PC usage. Accountant types could amortize that part out of the hardware purchase cost.