IIRC, the clear DI filter was $129 when I bought it. The DI CPL filter-only was $250 and the DI vND filter-only was $360, meaning buying the vND adapter and the separate CPL filter was the cheaper option (but only by $10).
Worth noting that you don't actually need the clear filter for the adapter. When I was investigating the Commlite mount adapter that I bought to modify for EF extender use (which was well after I bought the clear DI filter), I discovered
this little $15 cover (
Amazon link) that does the job, and more importantly for me saves space in the camera bag since I only need one of the DI filter canisters, not two, when bringing both the CPL and vND drop-ins. Unlike the supertele lenses that reportedly include the clear filter in the optical design and thus require something to be in the slot, the EF-RF adapter has no such need so a simple cap for the opening is fine.