I think it should probably cost 3 kudosu. Maybe 2.
It seems to me, one is being more generous with their kudosu points if they use them to star other's maps. Why not reward this behavior? If you make the cost 3, then it's the same as if you starred your own map, mathematically.
Cost to star your own map with +2 priority = 3kudosu
Cost to star someone else's map with +1 (free)
with +2 (3kudosu)
Maybe if there is concern people would abuse the system, setting the cost to 3 for someone else's map would actually make it less value than if you stuck with starring your own maps. If you shot 2 extra stars at someone else's map, Their priority becomes +3, while if you shot those stars at your own map, you would've gotten +4, and the more kudosu points you go through on one other map, the worse it gets, since each kudosu star is worth +2 on your own map, but just +1 on someone else's.
Costing 4 or 5 kudosu for each extra +1 priority on someone else's map seems way too high.
