With regards to Bentonite application rate, Natural Waterscapes suggests "The application rate for bentonite ranges from 2 pounds to more than 6 pounds per square foot depending on the soil characteristics." If you have some decent clay it will be on the lower end. 5000sf would require 1,000 to 3,000 lbs by their recommendation to do the whole pond. A 2,000 lb supper sack costs $1,1175 from them. You may find it cheaper but gives you an idea of what it would cost from one supplier.
I chose to mix it 3:1 with decent clay and spot treat my pond with a 1-2' compacted layer where it appeared we were making connection to the water table. If you do have a water-table pond then you might be able to find where some of the water enters and exits. These are hard to plug but I'm sure M. Otto and M Gray have written about it in PondBoss Magazine.
An alternative is to use a polymer sealant like Soilfloc, but I think this only works if water is only flowing out, not in.
Good luck, and some of the folks that do this for a living might chime in.
