Anyway even if a watchtower costs 2000 it remains forever, so in the long run is cheaper than a spy.
I disagree that its longevity makes it a better option however.

Watchtowers become redundant when the front moves on, so you have to build more and more, sending your FMs on errands when they could be doing something more useful. And splashing out 2K every turn or so can hurt quite a bit more.

I can use the same 5-6 spies to monitor hotspots, moving them as the front moves. Should the upkeep costs ever become crippling [ never have ] I can send them on a mission to infiltrate a city which is usually a death sentence. The flexiability of the spy unit makes it far superiour imo in bang for buck.