At $180 rental cost per week, this hydraulic post hole auger looks pretty hard to beat.

Generic comment: I used to take pride in buying "specialty tools" and scorned the idea of renting. Now I have a barn full of "wonderful things," have sold off most of the powered stuff, gave it away actually. :-( Owning something is great, however, if it has a gas engine and is not used reguarly that's a major headache given the "mystery" gas we have to use. Now when I need a pressure washer or log splitter once or twice a year, I rent it. If something breaks, they fix it, if it's your's and you're busy using it, the world stops while you order parts and wait. If you're not comfortable doing your own work, add more time and money to the wait.

Post hole augers generally will handle six or nine inch bits, too small for your requirements. Given the size holes you require, I'd guess you're talking about a large and very powerful machine.

Did a fast web search and came up with this, no idea if it's bad or good, but for $4,000 not including the auger's would seem to make $118/day look good to me.

Good luck,

@exmar, your post did not include a link, just to let you know....

The machine rental I linked in my short comment above has about a three-foot depth and the bits can go up to 16" wide. It looks like I am close to my depth requirement, and the width I simply will not find on an auger that is not on a Bobcat. I think that is okay. Those counter-balanced single-operator designs done by companies like "Little Beaver" are really wonderful machines. The operation looks much more stable than the single or dual operator devices that lack a stabilizing bar and a counterbalance.

To put a photo to this, this design looks safe and more than sufficient for my small project, and I can rent it at $180/week:

OOPS!

http://www.constructioncomplete.com/ground-hog-hd99-honda-hydraulic-one-man-towable-earth-drill.html

Depends on the number of holes you are going to dig ? I have personally used the latter 2 man unit and completed 40 3 ' holes in a weekend easily . The unit was a rental for under a hundred bucks .