Cost is going to depend on several factors, including, but not limited to:
1. Location - always a factor in construction costs due to travel time for the contractor, parts for repairs, parts for maintenance, fuel consumption costs, refueling costs, etc.
2. Soil type - soil type will depend on what type of equipment is selected, which will further affect the price. If it's a sandy loam type of soil that is relatively easy to dig, then cheaper equipment such as dozers can be implemented. If it's more of a clay mixture, then you'll probably look more into scrapers. With the size you are talking about, odds are that scrapers will be the most efficient in the grand scheme of things.
3. Natural terrain - how much soil are you actually going to move? 10k yards is one thing, 250k yards is completely different. The amount you need to move also will depend on what type of contractor you want to hire, because mom and pop outfits with a D-6 won't be able to move large quantities of dirt.
4. Vegetation - what type of vegetation do you have on site? Low brush and scrub trees are one thing, but if you have a bunch of larger oaks then you are talking about a completely different story and a different cost.
5. Regulations & Permits - from a guy that is a contractor for a living, permits just suck. They are expensive and take about 5x longer to obtain than what anybody tells you. That means if an agency or engineer or contractor tells you that it takes about 2 weeks to get a specific permit, plan on it taking 10 weeks. And you are probably looking at anywhere from 5 to 10 different permits that will be required.
You are talking about a relatively large surface area of water with your initial size. Really to get anywhere near an estimate without knowing most of the details is nothing more than a WAG on anybody's part.