726*5 =$4356
1,206*3=$3618
That means the 3 gpu rig costs 20% more.
Not really fair when you pick different quality PSU for the rigs, especially a, IMO, lower quality & cheaper one for the 5 card setup. An equivalent Corsair 1200AX would cost another US$40 more, closing the gap by US$120.
Also you're getting a required high end board only at $145 because of a special offer, which may not be valid tomorrow for all we know, so let's throw that in $50x3 difference.
Since we're using risers here, no reasons why we cannot make use of 1x PCIe slots as well. Boards which have 3x PCIe slots of any variety are commonly available on Newegg for $100 or less. So that reduces the 5x rigs by $27 each or $135 difference.
Now add in the time spent to purchase and assemble the open rig frame, say 2 hours average each at a labour cost of US$10/hr for $60 total.
During that 6 hours, the 3-gpu rigs would had been mining away at some 4500~5000MH/s which could possibly net up to 1BTC, not a lot but if there was a lucky streak during that 6 hours, it could be more.
Adjusted amounts would be US$4221 vs US$3948, ignoring possible gains from mining first, which is less than 7% difference
The 1200w psu I picked out isn't bad I mean I picked out an 80 plus gold and cooler masters isn't a bad company. The corsair for the 750w I chose is only 80 plus(not gold) so they may be different brands but they are both fine.
I agree you could go cheaper with the the 5 gpu mobo I just chose that because it is the mobo I use.
I honestly consider bitcoins a hobby and I wouldn't consider the 2 hours building the rig wasted. Also if you learned how to build the rig once it would drastically take down the build time for the next two. You could easily shave 2 hours off that time. Another thing to consider is you could build all 3 rigs at the same time and it would cut down on transition time between steps. I bet if you did all three at the same time it would take no longer then 3 hours.
I'd also like to point out build time for putting the pieces in the case and relocating the wires so it stays cool takes time too for the 3 gpu cases. There's 5 of them to put together as apposed to three so I bet it would take even the most skilled computer builder at least 2 hours to put together 5 computers.
This makes the gap even larger again (too lazy to recalc the numbers) but I bet it would be closer to 15%