Big O, I have the same problem only my pipe through the dam is the culprit it has gone bad after about 33 years of service. My contractor has told me between 5 and 7K, this includes digging out some silt in the shallow end as well as repairing the dam, it does not include the pipe for the new over flow so your price sounds reasonable. My dam is 410' long and probably 100' wide at the base. My sink hole is right above the pipe through the dam, from the top of the dam down to the pipe is 55'. So I have quite a challenge also.
