Are you sure the basin is fine? I'd repair the leak when the pond is low and before trying to keep it full with water from the well. I would go with clay to fix it, but you have to know where the leak is, or blanket the whole area. The clay has to be installed correctly (read that as compacted correctly) for a good chance on it fixing the problem.
I'm not being "smart" but unless you can tell us how many gallons it's leaking out in a 24 hr period, my answer is you will need as many gallons going into the pond as is leaking/wicking out, plus what is evaporating, plus what the plants along the pond are transpiring, plus one gallon.
My pond is dug into the water table, in sandy soil. I can run my well 24/7 and when the water table starts dropping, it will just barely pump enough water into the pond to keep it at the level of water when I turned on the well. I cannot raise the level of water in the pond once the water table starts to drop. My well puts out 25 to 28 GPM, and I run it thru a 3" pipe roughly 150' to 200' to get to the pond. The ground slopes down gradually to the pond from the well, so there isn't much head pressure at all.
Your costs will vary according to how big of a pump you will need, what your electrical rates are, and how long you have to run the pump. Running mine (3/4 hp) 24/7 most of a month will cost me around $100 to $125.
