Derek,
This post got me curious so I did a little math. Here goes. 1 acre excavated 12 feet deep with 3:1 slopes equals 13,301 cu. yds.
1 acre excavated 10 feet deep with 3:1 slopes equals 11,867 cu. yds.
1 acre excavated 14 feet deep with three sides at 3:1 slopes and 1 side including digging out 3:1 to join existing acre equals 15,512 cu. yds.
In this area the nrcs has a county average. On excavated ponds it is $l.48, probably cheaper than there.
Using that price on the proposed depths:
12 feet deep = $19685.48
10 feet deep = $17563.16
14 feet deep addition = $22957.76
