The New York Times has a very interesting analysis on the higher costs gay couples (or unmarried heterosexual couples, in some cases, though they of course have the option to marry) may face for things like health insurance for a partner, having a child, etc. Their calculation for the worst case lifetime loss? Almost half a million dollars! Of course the numbers will vary depending on each individual situation, and same-sex couples fare better on federal income taxes because without their marriages being recognized, they aren't subject to the "marriage penalty."
Here's one example:
In our worst case, the lower earner’s employer did not provide health insurance and her partner’s employer didn’t cover domestic partners. So the lower earner had to buy coverage on the private market, while the higher-earning partner provided coverage for herself and the two children. All this cost the gay couple $211,993 more than their heterosexual married counterparts, who were able to take advantage of the higher-earner’s family coverage.
In our best case, health coverage cost the gay couple $28,595 more. We assumed both gay partners were eligible for employer-provided coverage. The higher-earner’s employer also provided domestic partner coverage, which covered her partner for the five years she stayed at home. When she returned to work, she used her own employer’s insurance.
Even though the couple paid nearly $29,000 more in premiums than an identical heterosexual married couple, it was cheaper than using domestic partnership coverage throughout because of the onerous tax implications, according to Mr. Williams of the Tax Policy Center. A nondependent partner’s coverage is taxable income, and she can’t use pretax dollars to pay the premiums, according to Todd A. Solomon, a partner in the employee benefits department of McDermott Will & Emery in Chicago.
But as the article points out, it may not always be about the money: the emotional cost of not having one's relationship recognized and validated is something you can't budget for.