Advisor is one thing. Consultant is another. In IT for example you will only talk with advisors, whereas consultants can also program or offer valuable insight for designing/implementing your website...
I know I answered from an IT point of view, but that's only because this is my field. Hope this helps.
The answer to all three questions is "it depends" I'm afraid :-)
For example my prices for consulting are vary between free and and a low four/high three figure daily rate - depending on who I'm talking to and why. Personally I never take equity. Other people are happy to. Depends what you're after and why.
I think the key question for you to have a grasp of before you go looking for mentors/consultants is "Why do I need them?".
Nebulous needs "startup advice" are hard for you to value, hard for you to find the right person, and hard for others to price.
Concrete needs like "How can I get 510K in the US for my UK companies new surgical product?" (hypothetically) are much easier for you to value, much easier for others to price, and much simpler for you to know whether the person you're talking to is the right one.
I do both IT and general business advising, as well as IT consulting - though I generally refer to both as consulting. However, my rates differ depending on what I'm doing, who I'm talking to, and so on.
For example, I can be brought in at the idea level, and help with structuring the business, finding appropriate help for the founders depending on what they need, work on developing a business plan and develop metrics for success. I also work with startups that need IT help, and help them figure out what they need, and how to get it.
Alternatively (or sometimes in conjunction with the above), I may do IT consulting, where I do much of the IT development myself or with other members of my team, as appropriate.
My rates for the advising portion are significantly higher than my rates for consulting. Part of the reason for that is that with advising, the client is paying for my time, while with consulting, they are paying for someone qualified's time. It might be me, but there's no guarantee of that.
If you want actual numbers, an hour of IT consulting will range from about $50 to $150 per hour, while an hour of advising will often work out to $500+ per hour.
I never take equity for any such work - I work to help YOU run YOUR business, not my business.
First: Ask yourself - do I really need them? Shouldn't I know what I am doing in my niche?