Gumboot,
At a bid letting 2/4/05 for a watershed between Eskridge and Admire Kansas, the bids on the 35,315 cu. yd. earthfill embankment were $1.30, $1.40, and $1.60 per cu. yd. Other bid items were clearing, pipe, seeding, sand drain, riprap, mulching and some other items. The highest price per cu.yd. bidder ended up with the lowest total bid.
I would use at least the 36" pipe. If I remember right the water level (pipe inlet flow line) should be 2 or 2 1/2 times the pipe size below the spillway (72" or 90") for the pipe to run full.
I like the 4 to 1 back slope. Takes more dirt but makes for a gentler dam that can be mowed.
My opinion of your bid is that if the contractor is reputable and the price per yard includes; coring the dam, compaction, saving back topsoil and topsoiling all disturbed areas above the water line, and he can get started on it soon enough to suit you, it's a good bid.
Those other items can add up.
I would recomend 2 or 3 60" anti-seeps. z
