NADam,
First off, my figure of $1,000,000 is a WAG (wild a$$ guess). You will need a lot more information to get a more accurate estimate.
However, if that number does NOT scare you, then time for more discussion!
One of the most important considerations for building a large dam on a river, is the potential loss of human life and property damages in the event of a dam failure. A 10 meter high dam would release a massive wall of water downstream if it fails catastrophically. Hopefully, there are no villages or even small homesteads in the floodplain downstream of your dam for a considerable distance.
The most common way for an earthen dam to fail in a flood is to be overtopped by the flood waters. Once a little channel starts eroding through the flat top of the dam, then all of the water tries to go through that "chute" and it can quickly cut through the entire height of the dam. The best defense against that failure scenario is to have a very large "emergency" spillway for floodwaters to pass around your dam. I assume with your large landholding that you can create a nice flat area at a slightly lower elevation than the top of the dam? A dense cover crop to use the root systems to prevent erosion would probably work well.
Also, do you have heavy machinery on your farm? (Like dozers and excavators?) Using your own equipment and your own skilled operators could save you a lot of money.
Just throwing out more ideas for you to consider.
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