septic problems

I was just wondering if anyone here smarter than me would be willing to help me along the way. I am I'm im the process of bidding an et bed in the next county over. I have installed lots of laterals but never an et bed. It is going to be roughly 33 x 50 and has to be excavated out 3 feet deep. I am comming up with roughly 184 cu yd. Is that close at all? Then I must install 12" of 3/4 washed gravel. I am getting about 92 tons? Then 2" of pea gravel. I am getting about 13 tons. Then 10" of sand. I am getting about 78 tons. Is this math close at all? If so dose anyone have any advice on how to price an et bed vs conventional laterals. Thanks in advance for all advice and criticism. That I will receive for posting this question as I am sure some one will say don't bid but hey everybody has to learn some how. I like getting an education its just I don't want it to cost too much