How do you spend money effectively? (I have 8000+ gp on floor 3 that I have no idea how to best spend).
Assuming you already do things like "buy out the Commerce Shrine to get a shop portal and sell your loot"...
Buy basic armor from the shop if you're missing any armor pieces after a floor or two. Don't overlook the Sunglasses; protection from changing light levels is handy, and you'd much rather have them on your face than a blindfold.
Buy basic weapon types you don't have. Got a cutting implement, for escaping? Rope, for capturing? A ranged weapon, for dealing with things you don't want to melee? A demolition tool, for breaking down heavy defences? Those four don't cover every situation you might want a weapon for, but they'll do for most of them. For completeness, I'd add 'a magic weapon, for fighting ghosts', as the fifth weapon type... but those aren't available in the Shoppe, so you'll just have to get lucky there.
Stash backup weapons of those types at the Summit, just in case your weapons get lost or confiscated. Consider upgrading to a better ranged weapon later, if you use ranged attacks semi-regularly and haven't found something good.
You can offer money at shrines for +Rep with goddesses (and minor temporary buffs). Every bit of +Rep is important, since this lowers the strictness of restraints and postpones possible punishments.
Buy items from non-hostile NPCs for +Rep with factions. It's best to focus on useful consumables if you're at all gold-limited, but just buy whatever if you have the cash. If you've got a high-ish rep with one faction, look for more of their people and trade as much as you can; becoming allied with a faction means you can recruit any of their people to your party at any time, which can be really handy.
Consider stocking up on quick-bind restraints; they're hotbar-usable consumables that can hit enemies who are Vulnerable instead of needing a Stun. The Blacksmith in the Shoppe stocks basic Cuffs that handily gimp fighter-types, and the Stuffing sold by the Kidnapper is a super-cheap way to interrupt casting and at least
stall most mages, so it's worth making sure you keep some on hand. Better quick-bind restraints exist (mostly cuffs and ball gags), keep an eye out for them.
Consider stocking up on rubber tape from the kidnapper's shop. If you've get the opportunity to use Truss 'Em Up in a 1v1, you've
usually already won... but for more complex situations, taped-up hands are one of the most effective ways of preventing an opponent from escaping their restraints, and they're available in bulk.
Make sure you keep enough high-end bondage items around to stock your cuddle lounge. Turning high-level enemies into party members takes a while, but it really helps.
If you don't spend every last coin you possibly can on everything that's ever for sale, you'll probably still have more money than you can reasonably spend after a few floors. This gives you a significant budget for 'decorative' bondage, if you're in the habit of collecting enemies. You don't
need to tie up
every girl... but if you're into that, you can probably afford to do so.