Help, Tentant move out, stain on the carpet, how much should i c

 Tentant move out, stain on the carpet, how much should i charge her.

 already done carpet clean service , can't remove it,

I don't know what to do, may be leave like that for now,

but want to charge the tentant some money for the stain,

need a surgestion for how much $ should charge?

see picture bellow

Thanks