Rebalance healers by making them cheaper.

They are rarely used now, this may help.

Original patch by: majima at https://gitlab.com/0ad/0ad-community-mod-a26/-/merge_requests/5.
Ported by: @vv221
Differential revision: https://code.wildfiregames.com/D4855
Comments by: @chrstgtr, @real_tabasco_sauce, @wraitii