I have considered spies a few times - but the cost of purchase and upkeep are way out of my range - when so many other units are so much cheaper.
The cost has previously dissuaded me from even trying them out.

I have played in about 25 games now and only seen spies used once. I took over an ai late in a game and the ai had built spies. Another player was using them too. They were not effective. They cost way more than they appear to be worth.

on my 1st map i tried them , and like you didn't think they were worth the bucks especially at the start... on my second map however that changed unfortunately toward the end of the game.. there were 3 player as allies on our side vs 3 allies on enemy.. i picked up comm traffic between them planning to take out 2 of us when they were done with current project... what struck me as fishy was they were only concerned about 2 of us and not the 3rd member of our allyship , a player who got repeated warnings from us he was in clear and immediate danger from them..and chose to do nothing.... so on a hunch i put one in his capital and....low and behold...he was playing both sides...i guess hoping to preserve his own skin probably thinking there was no way me and the other ally both low level players stood a chance against 3 extremely high level experienced players...hell if truth be told we weren't sure either..but we weren't going down with out a fight..so we hit them first...took all three out...and took out the little traitor out on our way back home.... i've used spys ever since..against foe and ally alike..you never know what you might hear ill usually end up with only 3 total,...move them around...checking on a concern here a concern there... idling one or all or reactivating one or all during a map depending on whats going on

oh yes you can.. you can move them around anywhere on the map ya want ..as many times as ya want.. only place you cant.. is a colony that hasn't been colonized you've explored or any colony you haven't explored colonized or not