Child support obligations are based on income, although the hearing officer's are given a great deal of power to adjust the amount due based on personal opinion. The personal opinion can include knowledge of assets and income or contribution to living expenses by other sources, like new spouses. Of course, how much the additional sources will adjust the payments depends on the gender of the payor.
College expenses are not the responsibility of the parents in most states. Although some states will require a divorced parent to contribute, they will first consider the child's ability to contribute through any available scholarships, grants, and work study. In other words, there is not always a definite right to have college paid for by anyone other than yourself. If your parents want to work something out they can, if one volunteers any payments he can, but it will be based entirely on his decision as to what is appropriate.
Outside the law, I believe only income should be considered. Assets are not generally used to live on and could be maintained for any number of reasons. If those assets are being held in anticipation of retirement and are accessed to provide for a child's education, shouldn't the child then be obligated to contribute to the parent's retirement support later on?
I know some people may disagree because of the necessity of a college education today, but when it comes down to it, your parents really don't owe you anything beyond basic support to the exent they are capable and don't owe you anything once you are an adult. There are resources for college and there's nothing wrong with taking out a loan for your own education.
edit:
Sorry, I forgot the w/o much detail. I guess I just wanted to explain my opinion against the title which automatically assumes parents should pay for college costs.