re: Can anyone estimate the cost of converting a ditch to culverts?

Be careful. Open ditches have much larger carrying and storing capacity than culverts. There was a subdivision in Broussard which was designed with open ditch drainage but the city allowed and even helped some of the homeowners put in culverts and close ditches. Really bad sustained rain events happen every two years or so. Well, the next really bad sustained rain put water in a dozen houses because of the bottlenecks and reduced drainage capacity.