Among many of the changes that are expecting to release in 3644 (that I sincerely dislike) is the Sonar change.
If you want to make the sonar more expensive at t3, then please make it so t1 and t2 sonar are able to move like t3 sonar can.
It feels like t1 and t2 sonar are just ignored straight for a t3 rush, since t1 and t2 sonar are not movable their limited range becomes only a defensive tool and thus people feel the need to rush a t3 even at the extra cost.
If t1 and t2 sonar became movable then the game would become a lot more dynamic navy wise, and the t1 and t2 sonar might actually find regular usage. (currently at earlier stages its just less hassle to spam out t1 air scouts)