Getting married can be free if you live in the right places.
Common law marriages are only recognized in certain states including: the District of Columbia, Alabama, Colorado, Iowa, Kansas, Montana, Oklahoma, Pennsylvania, Rhode Island, South Carolina, Texas and Utah.
In order to qualify as a common law marriage, you and your partner, in general, must agree either in writing or orally to enter into a husband/wife relationship and actually hold yourselves out as husband and wife and acquire a reputation as a married couple. If you and your partner live in any of the states that recognize common law marriages and do not want the state to consider you as married, you may be a candidate for a contract that would establish your relationship as cohabitants. This is important since a common law marriage is treated no differently than a ceremonial marriage, which would therefore entitle the surviving spouse to receive statutory benefits or a share of the deceased spouse's estate in the event of death, even if that's not what you had anticipated.
The cost of a wedding ceremony, of course, is a whole other ball of wax.