Although Rapunzel is a fairly popular tale, there aren't too many actual folktales that are considered "Rapunzel" tales. In Surlalune's collection, there are only 10 Rapunzel tales, followed by 37 more general "Maiden in the Tower" tales. These vary from folk tales to myths to legends, like this one from Azerbaijan. There is a Maiden Tower in Baku that inspired many tales, including this story, published in 1911 by Abraham Valentine Williams Jackson:
The tower entrance
This story, as Heidi Anne Heiner points out, is like a tragic mix of Rapunzel and Donkeyskin. Although the wikipedia entry doesn't cite incest as one of the reasons for the maiden committing suicide from the tower, but a woman being forced to marry against her wishes. Either way, the legends all seem to agree that something horrible caused a young woman to use this tower to commit suicide; a darker alternative to the traditional Rapunzel tale. But when you consider what it must be like to be imprisoned, alone, inside a tower for who knows how many years, it makes sense that the young woman might decide to take more drastic measures.
Although this tale isn't considered a possible "origin" for the Rapunzel tale (it's clearly descendant from Basile's "Petrosinella" and Charlote-Rose de la Force's "Persinette") I love when fairy tale-like stories are linked to history. Europe is full of mysterious towers, many of which were really used as prisons, so it sparks the imagination...