A taxpayer may claim a deduction from net income in an amount equal to the payments made by the taxpayer into the education trust fund pursuant to an education investment agreement or prepaid tuition contract under the Education Trust Act [N.M. Stat. Ann. Chapter 21, Article 21K] in the taxable year for which the deduction is being claimed. The amount of payments made on behalf of any one beneficiary that may be deducted shall not exceed in the aggregate the cost of attendance at the applicable institution of higher education, as determined by the education trust board. Married individuals who file separate returns for the taxable year in which they could have filed a joint return may each claim only one-half of the deduction that would have been allowed on the joint return. Individuals having income both within and without this state shall apportion this deduction in accordance with regulations of the secretary.