1.The most cost efficient thing for post-battle HP recovery is "Potion" (smallest amount of gold for 1% of HP)
2.The most cost efficient thing for post-battle MP recovery is "Beer" (smallest amount of gold for 1% of HP)
3.If you plan to resell things that you bough with your insignias in the Spire of courage, then "Steel armor recipe" is the best investment (best (gold from reselling)/insignia ratio).
4.If you have finished all personal quests with Chiyo, then you will be able to craft items in her room for free.
5.You can remove all negative stats, restore HP, restore MP and revive your fallen comrades FOR FREE by sleeping in the royal bedroom.