The designated costs of issuing the refunding bonds may be paid by the purchaser of the refunding bonds or may be paid from any other legally available source, including the general fund of the local agency, other available revenues of the local agency under the control of the legislative body, the proceeds of sale of the refunding bonds, the interest or other gain derived from the investment of any of the proceeds of sale of the refunding bonds, any other moneys in escrow or in trust or any combination thereof as the legislative body may determine; provided, however, that any amounts paid by the local agency other than from the proceeds of sale of the refunding bonds or from
interest or other gains derived from the investment of such proceeds shall be added to the total net interest cost to maturity on the refunding bonds in determining whether the test of the second sentence of Section 53552 has been met.