Better Accounting for Medicaid Costs Act of 2017 This bill amends title XI (General Provisions) of the Social Security Act to require the Centers for Medicare & Medicaid Services (CMS) to follow rulemaking procedures with respect to certain Medicaid subregulatory policies. Before issuing a Medicaid subregulatory policy, CMS must determine the extent to which the policy is projected, with respect to each year in a 10-year period, to increase state or federal expenditures under the Medicaid program. If the policy is projected to increase expenditures by an amount exceeding a specified threshold, CMS may implement the policy only through notice and comment rulemaking.