I would recommend getting multiple estimates and also using the pond builders / contractors as a resource for ideas. Ask them lots of questions. Most of them would or should somewhat be familiar with the soil in the area. When I had my pond re-done I noticed that the prices varied quite a bit from one contractor to the next. Some will just eyeball it and throw out ball park numbers, others will take samples and really get involved in the process. It's nice to hear what all of them have to say and then develop a game plan accordingly.
I would also not count on being able to sell the dirt. If you can sell it then that is nice, but you must also factor in the cost to load the dirt, haul the dirt and unload the dirt which for to me is not worth it. Fortunately in my case I use the dirt as a barrier wall to the neighboring property and it works out well.
