The vast majority of prostitutes are open to negotiation. Not every one on everyday, but there is absolutely no harm in asking, just do it politely.
If they agree? Great. You've saved yourself some cash.
She disagrees? Fine. See her at her stated rate if you want. If you're not desperate, leave it (her loss) and try again another day. Just because they decline one day, doesn't mean they will the next (rent approaching, business is quiet, electricity bill needs paying, need a fix).
She has her head up her own arse (sienna_bronze), spits her dummy out, and blocks you (yeh, great business sense
) then just change your sim/get a new email...and ask again