Slash It Ultimate

You will need 41.9k gold for unlocking every song and category (100+200+300+400+500+600+700)x15 -100 since the very first mode will be unlocked already at the beginning of the game, therefore you will need additional 8.1k in your hand to unlock this one.

I found "Words" mode is the most efficient one to farm with "Hard" mode. Go to Niki Kofman - Tropicom (or any not very easy song) and farm the Words mode on Hard.

A little additional tip is; aim for the accuracy. The higher it is, more stars you will have by the end of the round leading you a higher multiplier for your score. Everytime a new line appears in this mode, until the first words appears, make a mistake on purpose and the line will disappear without harming your accuracy rating. This may take a little time to get used to (you have to notice when the line appears to immediatly react).

Also for staying sane, mute the song since you will have to farm it for a while.