You have to include launcher costs: either an SSGN like the modified Tridents, or the SSNs that carry them. Plus, with an aircraft, you can use it multiple times, and often in unique and unexpected ways. A missile, not so much.
Plus, TLAM isn't expected to be very survivable in a contested environment, especially around S300/S400 sites. That's the impetus for developing the troubled JASSM.
