My husband grosses $55,000 a year and pays $860 in child support for 2 kids. When his daughter turns 18 it only goes down 30.00? Can that be right? 

It could be correct if the child support order takes into consideration one of the children becoming an adult (18 years old) but perhaps also requires him to pay something towards college or vocational school costs. Perhaps your spouse can ask for a modification of the child support order if his income is insufficient or change of circumstances, or age issues with respect to the children's needs.

Here is some information for your review.

40-6A-102. Definitions.

As used in the Uniform Interstate Family Support Act [ 40-6A-100 NMSA 1978]: 

(1)     "child" means an individual, whether over or under the age of majority, who is or is alleged to be owed a duty of support by the individual's parent or who is or is alleged to be the beneficiary of a support order directed to the parent; 

(2)     "child-support order" means a support order for a child, including a child who has attained the age of majority under the law of the issuing state; 

Here is the child support obligation statute:

http://www.hsd.state.nm.us/csed/guidelines.html