I've reached opposite conclusions - TMD being really cheap compared to the cost of TMLs!
I kinda dislike the fact that a turtle-type player can rely on very few TMD, whereas a player with map control needs many TMDs to protect their economy; but over time I've found that, provided an appropriate amount of resources have been dedicated to economy, the 'I have the whole map' player still tends to be able to defend against TMLs with a stronger economy, compared to the turtle.
OUTSIDE of such extreme turtle gameplay, I find the cost of TML and TMD to be pretty good - if TML were more expensive, I'd struggle to see any real use cases for it.
That being said, I'm talking very much from a 1v1 perspective. I thought that team games would shut down TML strategies even harder (since there tends to be less area that an individual player needs to cover with TMD), but I'm not experienced enough to anything more than 'reckon' here.