Zero
It opens showing Integra's fascination with the fairy tale romance. But quickly cuts to the scene from the series during the flashbacks where Integra is a child and being chased to be executed. But she finds Alucard, he quickly decapitates her pursuers and pledges his loyalty to her. She seems to feel obligated to express her appreciation with her body. Alucard seems slightly embarrassed but continues and deflowers Integra. Immediately afterward Alucard saves Integra from a bullet and the scene cuts. Turns out this was Integra's memory of how she got with Alucard, she is an adult now and sitting lazily on the bead, presumably after a bout of love making. Meanwhile Alucard has gotten up and got dressed. He beams her a crooked smile and she remembers he past.
Approximated Worth: $20
Not especially rare and no where near a rare coupling, however this doujinshi does take place during a certain moment of the series, alternate timeline doujinshi that fit within the time constraints of a series usually fetch more, plus the art is pretty good.