By the time you add up all of the variables, none of the possible solutions are going to be all that much different in price, unless you can salvage materials. A culvert is gonna be a couple hundred, but so would a bridge and so would bringing in rock for a ford. For me, the easiest and cheapest solution would be a bridge built out of timbers, but that's because I have ready availability of logs that I can cut timbers out of with the only expense being my time and gas to run the mill. That's not an option for most folks though.