A tax is imposed on the privilege of transferring title to real estate located in Illinois, on the privilege of transferring a beneficial interest in real property located in Illinois, and on the privilege of transferring a controlling interest in a real estate entity owning property located in Illinois, at the rate of 50? for each $500 of value or fraction of $500 stated in the declaration required by Section 31-25. If, however, the transferring document states that the real estate, beneficial interest, or controlling interest is transferred subject to a mortgage, the amount of the mortgage remaining outstanding at the time of transfer shall not be included in the basis of computing the tax. The tax is due if the transfer is made by one or more related transactions or involves one or more persons or entities and whether or not a document is recorded.