Overlaying an already properly designed road can be super cheap, but it requires the original road to have been properly designed. Whatever that cost might have been.

Not really. Rail is paved to high standards, roads to the same standards are more expensive yet. If I were to replace my driveway with a rail I wouldn't use the same high standards that the main line gets and so my costs would be much less (of course since there is no rail down my little street that would be pointless). Mainline rail is expensive, but so is highway paving.

No, rail is always more expensive, even at the lowest spec'ed design loads. It becomes more cost effective the heavier the loading, not lighter. But AREMA ensures it's pretty much always more expensive.