Two questions:
1) How deep is the water gonna be in the area where you are considering a land bridge?
2)Is there a source for cheap quarry stone nearby?
If the pond is not too deep in the area you are considering a landbridge and there is a good quarry nearby, you could consider building a portion of the landbridge through the middle of the pond with large quarry stone such as riprap or b-stone that would allow the water to pass through without a culvert. As long as the stone is crushed as opposed to river or creek run stone which is rounded and smooth, it could be placed at a 1.5H:1V slopes to reduce the quantity of stone needed.
Lets say you build the middle 100 ft of the landbridge out of quarry stone and the water is an average of 10 feet deep in the portion you use the quarry stone....for a 12 foot wide roadbed, you would need 100'L X 10'D X 27' effective W = 27,000 cf of stone needed or 1000 cy. 1000 cy would probably be approximately 1500 tons of crushed stone, depending on the type, size and gradation of the material.
A couple of things to keep in mind:
1) You would have to also have to buy several tons of well graded 1 1/2 or 2" crushed stone to "choke-off" the top of the big crushed stone to make a smooth base for your road bed.
2) If there is limestone available nearby at a reasonable price, consider using that rather than sandstone or other material. Limestone has a tendency to give water a pretty blue-green color; could be a nice second order benefit to your plan!!
