Wouldn't cost of burying that cable be like 50x of that anyway?
If the conduit already has a phone line in it that isn't being used, sure.
Do you happen to know, for these long conduits is it typical to do a solid run or to put access panels at regular intervals? If you're running a new cable are you actually trying to run a 1km snake through a pipe, a 100m snake 10 times, 50m x20, or something in between?.
I've never seen anyone run a 1km cable, but I've seen buildings be rewired with tens of kilometers of new cables a couple times.