Missiles.
Any mech designer who thought about this weapon type would instantly associate it with the words 'powerful' and 'expensive'.
Missiles did not necessarily have to be powerful, but they could be so long as their warheads were filled with highly volatile exotic substances. There was an endless variety of unstable materials that could produce huge exothermic reactions under the right conditions.
In fact, different human powers had managed to discover so many effective combinations that they had to be deemed taboo in order to prevent their excessive destructive power from depopulating entire cities and planets!
If the law did not hold back the use of powerful missiles, then their cost most definitely acted as another restraint.
Transphasic missiles were especially costly as all of the phasewater put into their warheads would essentially be gone forever!
