denog, for the following comments I'm assuming that you're curious about the cost of property for income tax related reasons. If that is not the case, just ignore my comments below and proceed with search of records at your courthouse, discussions with title companies, etc.
If the person that originally purchased the property has since died, the cost basis for tax purposes is no longer the original cost, but rather (in most cases) the fair market value at the date of death of the original purchaser. If intervening owners have also died, cost basis is determined by the fair market value at the later person's death. If the decedent's executor filed the federal Form 706 the property description and fair market value information will be shown on the form. If they did not file a Form 706 (typically because the value of the entire estate was lower than the amount required to cause filing the form) you will need to do some research as to the estimated fair market value at date of death --- appraisal district info and real estate tax bills, comparable sales near in time, appraisals after the fact from qualified real estate appraisers, etc. In some cases, where an executor elected to use the "alternate valuation date", the value is determined not as of the date of death but rather 6 months later.
That should get you started. Depending on circumstances of your case there are more issues to be concerned with, such as if the land was co-owned, or was held in a legal entity versus directly by a decedent, or if special use valuation was used for Form 706, or if the land was gifted by a living person to some other person, etc. You might have to get with a competent estate tax attorney and/or CPA.