A shared life is probably cheaper than two separate lives. Two people paying for one home tends to be cheaper per person than one person paying for one home. If you both are going to an event, then you only pay for the gas that goes into one car instead of two cars. Insurances and taxation might vary from place to place as to if they are cheaper per person as a couple.

She might also have a different lifestyle that is overall less expensive (ex. more frequently cooking at home instead of eating out, shopping at cheaper stores).