To save more gold in the long run make sure you have enough space to collect manyu's and only feed to a girl when you can feed 10 at a time. The only exception to this rule is when you are near level cap and don;t need 10 to get there.

Best feed rate for lvling girls is as follows:

10 x Lvl 5
10 x Lvl 5
10 x Lvl 20

And continue with 20s and 100s if you have them after that. those 20 lvl 5s should lvl your girl to lvl 29-31.

Also late game the 80 stam GOLD mission is the best for getting just gold. Nothing in game yet can beat it.