Similar to other spanning tree protocol configurations, the cost and port priority of Multiple Spanning Tree can be tweaked.
The command is different though to tweak the cost for Multiple Spanning Tree:
spanning-tree mst X cost Y
Substitute X with the instance number of the Multiple Spanning Tree process and Y with the chosen cost.
The command is different for port priority too:
spanning-tree mst X port-priority Y
Substitute X with the instance number and Y with the priority.