in the long run, it's a small amount to pay for peace and tranquility for the rest of your life.
File the papers. you can stop his spending with a divorce action. then you just add up the assets, subtract the debts, split the difference. He has nothing to lose by suggesting you pay more.
count on him being the unreasonable one. You should try being unreasonalble too. Accept nothing more than paying 50%. Lawyer up. It's cheaper in the long run. With no kids, the whole thing will be so much cheaper in the end. Fret not.