I run a web hosting company and have lots of experience cleaning sites up. Very likely your site has been compromised by malware, resulting in the redirections.
I charge $30 per-site for average-sized Wordpress sites and that's the most common site, so that's what I'm quoting. More complex sites, such as sites containing other sites and woocommerce will cost more.
Part of my task is to replace Wordpress, scan the active theme and reinstall all plugins.
If your Wordpress install is very old, there's a chance a plugin will misbehave or that a design issue will pop up due to the updates.
These issues will need to be addressed by a designer/developer and falls outside the scope of this bid.
I advise that you delete any unused plugins, once the sites are clean and secure.
Cleanup time will depend on how many plugins you have, but I should have your site back up 1-2 hours after the job is started.
To do the job, I will need the following information:
Domain affected.
Hosting panel login/password.
Wordpress admin login/password.
SSH admin/password (If available)