Short Equation: Shinobu becoming a Prostitute 97% = $0.75 [Nightly cost[) - $0.65 [Brothel fee] = $0.10 Whore [Shinobu's worth].
Long Equation: She's a daughter of a Duke, who is spoiled and gets what she wants. The Duke is high up, yet he can't go against The Countess, who has a big voice within Society. Even though it's the Nobility's money, mainly the Duke's, that is keeping the Chamber afloat, without the Soldiers and Yako, the people are basically Ayakashi food. With Yako only in love with Nanao, the Countess taking a liking to Nanao, the soldiers too...couple that with the women of Nobility reeeally being done with Shinobu's bullshit plus the Duke having to keep up with appearances (meaning he can't take his daughter's side, even if he wanted to), Shinobu's chances of becoming a Prostitute is at a 97%. She will probably make 3 silver coins. Let's say...75 cents. Of course, that money goes back into the Brothel so, she might see 10 cents of that. That enough to buy a half loaf of bread. All in all, she would basically be a '10 cent Whore".
So the chances of her even being an Oiren was nill
*applause*
I don't think ML's mom would handle the repercussions of taking out one of their best fighters against any pissed off youkai...
And as for Shinobu...
Well, while being tainted is a preferred comeuppance, the fact that her reputation is practically a shit hole is a far worse scenario. Also lowering her chances of EVER getting with a guy who would give her a pity date.
...QUICK! Someone calculate the possibility of her being a prostitute and how much she would make on a daily basis? Compared to those that work in the Red Light District.