In the production documents of season 6, John Saxon was considered a regular. Therefore, he was not listed in the usual sheets about "supporting cast billing information", which were per-episode listings for various purposes, such as for post-production (i.e. for appearance and position of the on-screen billing) and accounting (i.e. for paying the talent). Instead, he was listed in the big ensemble cast sheet among the regulars.
I know from the documents in our archives that John's salary was above Brett Cullen's (who made it to the main titles) and a little less than Margaret's.
Keep in mind that compensation for talent is a mixture of various aspects, not necessarily the salary alone and not the position in on-screen billing and terminology (such as "special guest star", "special appearance by", etc.) alone. Compensation is usually something the talent's agent and the production company negotiate. Some actors and actresses prefer top billing and care less about the money, others demand more money and care less about the position of their name. It's a matter of negotiation.