Reworking the pond would include breaching dam to drain the pond. Removing all trees from dam and rebuilding it - might require a new core. Then comes the excavation work to add depth/structure to your pond.
My WAG would be $10k for the dam work.
Figure excavation at $175/hr or $1.75/cyd dirt moved. Think you'd be better off renting the equipment and moving dirt yourself - excavation should be much easier than dam work IMO.
I think I'd prepare for at least $15 - $20k at best. I would review that bid with the land owner and see if there's some room to negotiate. Maybe he would offer to split with you and drop the price $10k?