I was wrong, it's actually 112,160 prestige to buy all cosmetics.

Or rather, it's at most 112,160 prestige, and also about a dozen of every boss material. Apparently they lowered the costs for some things in the most recent update, meaning if you're playing the game now it'll take less to get there than it took me.

My previous count from an earlier thread missed some stuff, because there's a section of the Crossroads I didn't notice could be upgraded, and because there's a few upgrades that take extra shenanigans to unlock. Thank you to u/Crestfallensolaces for finding my mistakes, and to whoever maintains the wiki for listing unlock requirements.

Anyway, the places you can buy upgrades are, as of the Unseen Update, the following:

Mel and Dora's tent: 32 decorations

Cauldron area: 45 decorations

Taverna: 28 decorations

Training grounds: 15 decorations

Music Maker: 43 songs

Most of the unlocks are gated behind things you'll do naturally while doing other Completionist Stuff. Clearing both routes several times, getting a few hearts of affinity for certain characters, getting every familiar, doing every Incantation, buying enough cosmetics to unlock the next tier of pretties, and so on. There are a few unlocks for things you might not think to do, though. Specifically:

-Beat the final boss of each route on Vow of Rivals 4 twice. You'll get an extra song per route after your second win.

-Salute the fountain behind the garden on two different nights. The one that looks like Hades's head.

-Pick up 10 rubbish from next to Eris.

-Catch 3 Ribeye (common, Ephyra), 3 Soby (common, Fields of Mourning), and 3 Goldfish (Rare, Tartarus).