Ok... so.. depending on how stable/reliable you want your system to be, a 4 card system can be dirt cheap (excluding the video cards). I can show you the current setup I have:http://www.newegg.com/Product/Product.aspx?Item=N82E16817101044
$31 (secondary PSU for 2 of the cards) (you can add another if you want 5 cards)
cheap 600W power supply ~$50 if you wait for a sale (primary PSU for the other 2 cards and MB)http://www.newegg.com/Product/Product.aspx?Item=N82E16813128451
$80 (5 PCIe MB)http://www.newegg.com/Product/Product.aspx?Item=N82E16820134784
$13 (1GB ddr3 RAM)http://www.newegg.com/Product/Product.aspx?Item=N82E16819103698
$38 (Sempron 140 CPU)http://www.9mart.com/products/2-Lots-PCI-Express-Card-Extender-Cable-PCI%252dE-1X-Riser-Adapter.html
2x2 $18 (PCIe extenders... shipping from this site is really flaky.. but most other places are over $20 a piece)
random hard drive.... i have no idea what the price of these things are... i just have a whole bunch laying around... (maybe $35?)
now this is the tricky part, you have to somehow mount the cards above the motherboard and either sand off or cut off the ends of the pcie extenders
this is what i did (for about $20 worth of materials from home depot to handle 10 cards):http://www.amazon.com/Sapphire-Radeon-PCI-Express-Graphics-100282XTREME/dp/B004W75ATI/ref=sr_1_2?ie=UTF8&qid=1305146844&sr=8-2
(5850s for $164 shipped)
so total for a 4 card system = ~$285 (~$315 for 5 cards) + the cards (total = $921) $230 per card or $1120 for 5 ($224 per card)
Now, I do warn you that making a computer case like this is probably not the best idea especially when those 10 cards pull about 12 amps from the wall... but hey, if you are a risk taker and would rather do things cheaply than do them right... Otherwise, you can always just add a case for another $30 or so, and take off the exta PSU so a 2 card system would be $593 ($296.5 per card)
If you are interested, I can either start a new thread with more details instructions on how to do all of this.
As for profitability, *sigh*... I've been going through this a lot and for the past few difficulty increases, the price of bitcoins has gone up proportionally to the difficulty, and I am getting around ~$8 per card per day (including power) for the past few weeks. Whether this will trend will continue is pure speculation... At the very least, you'll make some money and you'll be able to ebay the cards (or keep them as secondary computers) when it stops being profitable.
Sorry, I like to go into a lot of this stuff... I just love bitcoins that much... haha. But if you have any questions, feel free to PM me.