They have increased CPs for them.
They won't do anything with the cost, because they don't overperform for the cost, if they increased the costs, they would have to buff them as well.
Increasing cost isnt the only way, theres loads. Tie it to building requirement (like Kappa patch), while you can still choose to stall for heavies, it is an indirect price increase if you do so (having to tech), and the simple fact its a requirement would mean you may as well get a medium or two anyway. It would shake things up a bit.