There are a huge amount of variables to consider to come even close to an approximate cost for a project that size, one of the biggest ones is how many acres of drainage do you have filling up a 50 acre pond, is it 50 acres now or will it be fifty when the overflow is raised? obviously a lot of acreage would be needed to maintain the water level which means a pretty substantial spillway system to handle heavy rains thru the wet season or an extreme rain event.
Has anybody estimated the yards of dirt that would be required to reach your desired elevation?
Like Bocomo said, a professional would be well worth the money in this instance because of the enormity of the project, the original investors would have been very well advised to invest in more research and design as it appears they threw their monies pretty much into a nuisance swamp. jmo