If you have a "silted in" pond, it would cost 4 times the price of bucketing out muck over cutting a breach in the current dam. 50K sounds about right for moving silt and rebuilding/expanding/raising a dam....it will take a LOT of earth being moved.
A siphon is by far the fastest, lowest cost method of draining a pond, but the total "lift" is a limiting factor. Using 4" (or even 2") schedule 40 PVC and rubber connection couplers makes a temporary siphon easy to build. Also, a complete renovation would be the perfect time to install a permanent, well designed, bottom draw, automatic start/stop siphon that will nearly eliminate future silt issues forming in the future. If you are prepared to spend around $50K, plan, plan, plan, and don't cut corners to save the most dollars!
