that's why i love Beastmaster against tinker.... he has a huge stun to deal with him, a boar that can disable dagger, a hawk that can scout in trees, and an ability that can destroy trees for the rest of the teammates.... all in one counterpick.... and yet people don't really pick it against tinker....
as for the other thing, increasing mana cost of dagon will affect other heroes too, not just tinker... unless icefrog thinks dagon is OP (i don't agree), increasing mana cost of rearm makes the most sense... also increasing mana cost of rearm doesnt mean jack shit if tinker simply skips the additional levels in rearm, which brings me back to my original point that something has to be done to force tinker to pick the additional levels....
one thing that can be done is changing the interaction between rearm and soul ring.... the soul ring effectively reduces the cost of rearm by a significant amount.... that should be looked at