i wouldn't go off buying F/O time at air carriers. Why? Its illegal.
First of all, you can only log the flight time, if you are a required flight crew member. If the flight requires a F/O, then it can only be an "employee" of the company. You are only "employed" by the company, if you receive compensation. So if you are paying to fly, you are hardly employed.
If however, the flight does not require a F/O, but the company lets you ride along anyway, you can not log this as flight time, since no crew member was needed under the operations certificate.
This is how AOPA describes it in their magazine.