I'd like to do a PhD in a field completely different from what I have studied so far. How plausible is it to get admission into a top school for a PhD given that I will have no experience?

The answer to this question will very much depend on what field you choose to pursue. I can speak to physics. I started my PhD program when I was 27. I worked for three years after my bachelors degree. So there is no barrier there because of age. Also, you could expect tuition remission, and a modest stipend. If you pursue theory, you will probably have to teach. If you purse experimental physics, it is typical that your advisor's research grant will cover your stipend. Top school? Probably not, without a degree in math, engineering, or a hard science, and/or research experience. You would be too far behind graduating seniors, and that would need to be made up. A better bet would be to work on a masters degree in a school you can get into, and then apply for a PhD program in a "top" school.

You didn't say a word about the field in which you want to do your doctorate! Ph. D. is about knowing very very deeply about a very very narrow topic. Would that even interest a guy like you? I think you'd be really frustrated with the narrowness of the scope of the topic (depending on the topic of course). It's no fun to be competing with really talented and focused students (with years of well-honed core subject knowledge) in your department when you know you are a newbie. I don't want you demoralised after a year or two into the program.

With your experiences and background I really don't understand why you want to do a PhD. Really, if you have the means to fund your problem-solving I would advise you to stay away from getting into a PhD - mainly because in most programs you hardly get to do exactly what you want... you will need to align your interests with that of your supervisor and other stake holders (like the university and funding organization) and after all the hullabaloo the only thing you achieve is recognition in the form of a diploma, that is of course apart from the knowledge that you will gain (which you could do without joining a PhD program). If thats what you are looking for then go ahead and try. I'm sure you can get into any top school unless its a very tech intensive field where several methods and techniques know-how is mandatory.

I recommend against getting a PhD if you've never worked in research. is correct in that getting a PhD means become an expert in a very very narrow field, so I'm not sure how you can want to get a PhD when you don't even know what general area you want to work in. He's also correct in that the topic you study isn't dictated by you, but by your adviser, who pays you (very poorly, I might add). The one advantage you have may be money. PhD programs are generally hard up for cash, so if you can fund yourself instead of the department funding you, maybe a prof somewhere will take you on as free labor. This is non-standard practice, so you'll need to convince someone this is a good idea. But still, I can't recommend doing a PhD if you don't know what you're getting into.

The bigger obstacle for a Ph.D. in the US is passing the qualifying exam.  You fail twice and you are out.  Since you are a successful entrepreneur, a Ph.D. in management or economics is possible, provided you spend a couple of years to take enough courses to pass the qualifying exam.  Then you can focus on a Ph.D.  If it is completely unrelated, unless you have a strong GRE and very good grades from your MS, it may be very hard to get accepted in a top school, although, you never know, some schools really love alumni who potentially can give them very good donations.

What field of study? Well, I'll assume you want to work in my field, which would likely mean you'd want to do bioinformatics as that is at the intersection of computer science and biology. First, you'd have to show aptitude in biological research, which means you'd have to work in a lab. Academic, government, or industrial is fine, but you this is an absolutely necessary requirement. A PhD is a research degree, you must have experience actually doing research to show that it's a good match for you. Since you don't have college coursework in biology, it would be very wise to take either the biology or biochemistry subject GRE and do well on it. It'll take a lot of work , but so does a PhD. Self-study using Khan Academy or Coursera is a good idea. Money shouldn't be a concern. You don't pay for a PhD in the sciences, you'll be funded through working as a teaching assistant or directly through your advisor. You'll also receive a modest stipend. $25,000/year is the norm, some schools offer less and some offer more. You probably won't be able to work much on the side as a PhD is already a full time job and possibly more (especially if you're doing coursework or TAing, and you'll start it deficient in vital coursework).