How about ontop of instancing, decrease the drop rates for GWD Bosses/Minions and add a fee of maybe 100k/min to stay inside? So click on the door, an option appears and you choose how many coins you want to put in, could be 100k/min so if you put 1m that's 10minutes. Once inside a timer appears and goes down for as long as you are inside the room. Leaving pauses the timer and you can resume by going back inside. Also you can stack payments so perhaps you pay 1m first, then decide that's not enough so you put in 5m more, totalling 6m which equals 1 hour.
Or have it so you pay for how many times the GWD Boss appears. For example, the minimum fee to enter is 200k which is 1 kill, so if you pay 1m to enter Bandos, then Bandos will appear 5 times (this includes his initial spawn when you enter). 200k x 5 = 1m.
You can even make it so each GWD Boss has different prices as each one may be harder/have more valued drops.
Literally just throwing ideas out there so yeah.