There's a lot of talk about the mining rig "jig" being up on the bitcoin forums themselves. I still think there's some room, and I will cut and past my post on there, as I feel it really helps the discussion here.
http://forum.bitcoin.org/index.php?topic=18565.0;all
I decided to get into the bitcoin mining business recently, and have other bitcoin projects going.
I should be coming online to about 3.2ghash's in about a week, this cost me $4400
The only problem I see is with scale, and your cost of electricity. But really, the electric only matters as to when u get to your breaking even point. Maybe I'm just lucky my electrics low, 8c a kw hr at peak
The way I see it, its 8 weeks to break even on equipment if bitcoins stay at $15-20 a piece.
And really, when it comes to "breaking even" you also have to factor in being able to sell the machines too. I'm sure i could get $500 per pc with a ATI 6970 in it (yeah i know i should have gotten a different cheaper card live and learn), but thats 8x500=$4,000
So, really I only have to make $400 before I "break even"
Now I got kinda lucky because i had 8 pc's laying around. But u can get a cheap $200 dollar old computer and pay the $550 I paid per cheap Pc (power supply, video card, I got some other stuff put in too that was unneeded). So say total you pay $750 per machine, that you can realistically resell for $500. But lets say you want to be conservative, and say $450 a machine resale on CL, eBay, etc
So your looking at $750x8 (in my case), and your getting 3.2ghash/s. Total you paid $6000. Selling them off fire sale prices you can get $3600 back meaning you need to make only $2400 to "break even", not $6k
3.2ghash's @ $15 a bitcoin @ 877226.666667 is $1,673.09 per http://www.alloscomp.com/bitcoin/calculator.php
$1,673.09 divided by 30 = $55.75
$2400 divided by $55.75 = 43.wholebunchofnumbers days
So if u wanted to step into the market today with $6,000 after 43 days u would be pure profit to the tune of $390 a week
Mind you, I've overstated expenses, and understated earnings and prices u can receive too make WORST CASE SCENARIOS
The matter is scale. 400mhash's is a piss in the pot. I only got 3.2g as a starter to see how it goes, but if it goes well I will be scaling.