Around here (Abilene Texas) a D-4 can be had for $265 a day. (I think the brush rake is an extra $20 a day though)
A small backhoe (like a john deere 110) run about $140 a day.
A regular sized backhoe (JD 310, case 580, cat 416, etc) about $180 or so.
There is only one place around here that has reasonable haul fees and/or includes a trailer. Most places here charge about $1.50 per loaded mile to deliver/pick up equipment.
They also charge you a very high price per gallon if they have to refuel the equipment. Gets tricky if you cannot haul the equipment yourself to fill it up. Personally, I have a red diesel permit, so I take a couple of drums and buy bulk and just refill myself... ALOT cheaper.
As for the septic tank, a 110 sized backhoe will not lift a concrete tank, and if it is a very big tank, or the ground is soft a full sized one might not either. Just becareful and you'll be okay!

Of course, if you're using poly tanks, then it doesn't matter anyway.
When calling for around for prices, be sure to ask how many hours on the machine they count as a day. Not all places charge the same. Around here the better places allow ten hours on the machine, some others as few as seven hours (for the same daily rate!!!

) The place I usually deal with are good people and don't sweat an hour or two over the day rate, some of the others try to charge another full day's rent... but I guess there are crooks in any industry.
Good luck with your dirt work!
Rowdy