> Hardest to write to I'd say, unless I just go full "It's a guro story, I ain't explaining shit" mode to write off why someone would remove their bones for money

If you need some ideas on how to write it, you could, as JIP said, have her be in financial trouble due to losing her job and need money fast to pay rent. You could start it out as having the character(let's say Amanda) lose a tooth somehow. It could then end up under her pillow somehow and in the morning Amanda could find five or so dollars. Being shocked, she could go find one or two other teeth that she'd lost in earlier years and place them under her pillow for the following night. After waking up the next day and finding ten or so dollars, she could get an idea to capture the Tooth Fairy in order to ask her for help. This is where she could start forcibly removing parts of her body. She could take several painkillers and, after much struggling, remove a single tooth. That night Amanda could place the extracted tooth under her pillow and patiently wait for the Tooth Fairy to appear. Eventually the Tooth Fairy could appear and Amanda could quickly capture her in her hand as she could be abut the same size as Fey is in this story. After finding out that the Tooth Fairy can't give her more money, she could angrily ask "Why not?". This could get the Tooth Fairy to explain that she and her entire race live off the solid calcium they extract from the teeth and that hey exchange money for the teeth they find under pillows. Amanda could then ask if calcium could be extracted from other sources(I'll give you three guesses what this other source could be). This could eventually lead to the Tooth Fairy blessing Amanda with extreme regeneration(except for bones, of course), along with allowing her to easily live through any injury, no matter the severity.

The next day, Amanda could start by removing the rest of her teeth, figuring she could just use dentures as a replacement. Doing this, she could note the absence of any real pain. That night, she could put the assembled teeth under her pillow and get over two hundred dollars for them all the next day. Slightly downhearted at the 'small' amount she got for them all(at least when compared to how much she needs), but emboldened by her success, decide to kick it up a notch by carefully removing the bones in her toes. The following day she could be shocked at seeing she got roughly the same as she had for all her teeth due to them being slightly bigger, but lesser in number. Getting desperate, she could decide that she could lose a few ribs and remove the two bottom pairs of ribs from her chest. The next day, Amanda could realize that she got significantly more for her ribs than what she had thus far gotten. Despite this, she could see that she would need to start making some sacrifices if she wanted to reach the required amount of money. Deciding she could live with being paralyzed from the waist down, she could remove her pelvis and everything below it, figuring she could explain it away as being caused by a degenerating bone disease.

At this point, you could start mixing in some kinky fun due to her regeneration and lack of a pelvis. Also, I've left out any real storytelling, so you could fit that in by yourself, maybe show how Amanda copes with missing some of her bones. Secondly, you could have Amanda's best friend show up at some point and join in on the fun. Lastly, when it gets to the point where all Amanda has left is her skull, the Tooth Fairy could be disappointed that Amanda has nothing left to give her, but brightening up, could say that Amanda has one bone left. This could confuse Amanda, but when the Tooth Fairy(you could also give her a name; I'll call her Lilly) flies into her ear and into her brain, Amanda could protest, but without any bones to stop her, Lilly could remove Amanda's skull, causing her face to deflate. Amanda's friend could say that it was probably for the best if she wanted to stick to her bone-degeneration disease story. From here you could move entirely to kinky guro without having to focus so much on story progression. You could even have Lilly come in too, kind of like what you did in this story.

Hopefully this gives you a few ideas for a Tooth Fairy story. =)