No, AI hero reinforcements via cf_reinforce_party for lord parties cost 300/450/600 depending on difficulty, if their size are below a certain ideal (see script_hire_men_to_kingdom_hero_party). Their ideal party size are determined by 10+(leadership*5)+charisma+(renown/25); if they are the leader an additional 100, with 20 more for being marshall (can be the same lord), an additional 20 for each castle (not town) they own, and a level-scaling multiplier which is (1+[player lvl/80]) applied after the above calculations. These are all in script_party_get_ideal_size.