A cost efficient culvert could possibly be made using 2-3 salvaged fuel tanks placed side-by-side with about 2 feet of space between each one. You cut the ends out of the tanks leaving about 4 inches of the end wall all the way around the outer diameter. If the tanks have alot of fuel residue in them, then pressure wash inside the tanks good before placement. Backfill around the tanks with sand and top off with about 6-8 inches of crushed stone or concrete. Good top cover over culverts is very important. Lots of farmers here in North Carolina have used salvaged fuel tanks removed from gas stations for culverts.