I looked into this just 6 months ago. Range is wide. I was asked to join as the 2nd board director (first was founder). We agreed to a monthly rate up to X# of hours I make myself available to the founder. Also 1% grant and 1% option. And a flat fee for attending each qtly board meeting. Those are the three basic components without getting too creative with performance-based metrics.

Advisory Board or Fiduciary? I'm self funded and dipping into my personal network of highly experienced executives to form an Advisory board so I can only speak to that structure: I've estimated approximately how much time per year I'll need including quarterly meetings and added a small buffer for ad hoc advice. Then apply a reasonable flat rate that when looked at hourly is befitting their experience. Additionally, there is an indication of the job performance bonus based on company performance. In the case of an Advisory Board, I strongly suggest you put together an official job description which includes an annual review for their 1 year term. Lastly, within the job description I set the expectation with my board members they are to help open doors and make introductions for the business.
A resource you might consider is the Private Directors Association. They provide a certification for board governance which I found useful, but not earth shattering.