Hello,
That's what I do on daily basis: I clean and secure compromised websites/hosting account, especially (but not only) Wordpress and Joomla.
I'll clean, upgrade and secure WP, not a problem :)
The price may vary and depends on how many sites your client have. If it's som kind of shared hosting account I'll have to copy all sites to another, secure server, clean them one by one and move back to original location.
Let me know if you have any question!
Regards,
Adam