Then dont spend. Have some respect for yourselves, if they give you bad rewards, dont pay and encourage them.

On topic:

myoboku gifts are propably the best and cheapeast way to increase power in general.

Charms, refines on first position, while giving you less overall power have a significantly larger impact in pvp. You could also pick the most refined gear and try to get its purification to the highest possible level. If you havent done it yet, you can max out mood on your main team (if you are using only 1 set of ninjas for pvp). Also look for tool essences, because they provide a lot of initiative+stat, good deal. I assume your mangatamas are all lv6 and runes around lv5~6 each, so other power sources (mangatamas, runes, purification in general, battle armor, summon cultivation, costumes) are rather expensive for the ammount of power they provide for team. It can still be beneficial to improve your first position even if it gets a little expensive, but at your current stage its not worth to do it on everybody else.

You're saying German version, arent tactics kinda op thing there? Could be worth looking into it.