Chandler has been shown to work for a big company, having his own office, always dressed up in a suit while at work and also having been assigned an office secretary. That job has got to pay well.
We know Chandler has never had a car and probably takes the metro to work. We also know that he’s supported Joey on numerous occasions when the latter didn’t have an acting assignment. Think about the time when he was looking to just give away $1500 to Joey so his leaving the apartment to go live with Monica doesn’t become a huge burden on Joey. Later on one occasion (S08E21) when Joey is angry on him because he fell asleep at the former’s movie premiere and out of spite wants to pay him back for everything he’s done, Chandler counts two headshots worth $1000 plus “acting classes, stage combat classes, tap classes” followed by “two three years of rent, food, utilities”. Remember they live in Manhattan where nothing is supposed to be cheap. Initially Joey seems alright as he types in $1000 into the calculator and then some but we see he’s both shocked and embarrassed looking at the final tally so that must have been around $20000 give or take. Chandler doesn’t get a cheque in the end since they reconcile and that doesn’t bother him the slightest.
Just before the plot where he writes down what he’s saved in the past six years (despite Joey owing him so much) we see him go ring shopping with Phoebe (S06E23) where he ends up paying freaking $8000 for an engagement ring.
Taking all this into account and extrapolating from the data we can safely say that he wrote down something inching towards six figures on that piece of paper. He stops himself from revealing it when he sees Rachel all concerned and only then starts scribbling it down. He must therefore know that none of the others could possibly have that much so flaunting his wealth in front of them might make him look bad.
Then there’s Monica’s reaction: