It's not normal to work for a company for free, and I'd simply answer something like this:

Dear X,

I am glad to know that my PhD work caught your interest. I can provide more information under a suitable consultancy contract.

Kind regards,

L222

Note: We don't know the content of the company's email and, as some have remarked, the above response might be a bit too sharp, especially if their attitude is not so exploitative as I assumed. A milder approach, which I like, is that outlined in Dan Romik's answer. Since only you can read the email, I strongly encourage you to consider carefully both approaches, regardless of the accumulated votes.

Other answers make the case that you should ask for money for your services. I certainly think that is a very fair and reasonable thing to do, but rather than asking for money upfront, I would like to suggest the following strategy to maximize the benefit (in terms of financial gain, experience, and networking opportunities) that you can gain from this situation:

1. Respond to the person who contacted you with a brief and polite email saying you'd be happy to have a short meeting (over the phone or in person at your office or somewhere nearby which doesn't necessitate any travel on your part) with them to give them a brief explanation of your work and/or hear more about the project. Do not ask for money or mention the possibility of paid consulting. I assume this will be followed up with an exchange of emails to set up the time/place for the meeting.

2. When you have the meeting, be open and genuinely willing to explain your work (or at least any details of it that are in your thesis or already published), for free, for a reasonable amount of time - say up to 30 minutes or an hour.

3. If you see that the conversation goes on for long enough that you are about to exceed your predetermined free consultation time, politely -- but assertively -- tell them that you will need to end the conversation soon, and say that you will be open to continuing the discussion at a future date, but that seeing as your time is valuable and you see that they need more of it than seems reasonable to offer for free, you will want this to be done on the basis of a paid consultation. Be ready with a figure for the hourly rate you want to ask in case they end up asking for it, but it's quite possible that this won't come up.

Following step number 3 above, either they will want to hire you to do additional consulting on the project, or they won't. I think the strategy above maximizes the chances that they will. Moreover, I should add that it is quite common for highly paid professionals (lawyers, financial consultants etc) to offer a free consultation before they start asking to be paid. I think the reason they do this is similar to why the above strategy is a good one -- it makes it easy for the client to approach the professional and explore the possibility of working together (by contrast, if you ask to be paid in the initial email this might be a turn-off and cause the people involved to simply give up the idea of talking to you, even if it may be in their interest to do so). The point of this comment is that I don't think you need to worry that you are being exploited by offering a small amount of your time and expertise for free -- even if you don't end up getting hired for a consulting gig you'll still have an interesting experience and sow some useful networking seeds that may be prove useful in the future. Good luck!

I've never worked with a commercial interest directly, but the impression I get from colleagues is that companies will take as much as they can and pay as little as possible. "It's just business" as they say.

Your experience is valuable, so I wouldn't give it away for free. Ask them what kind of consulting fee they're offering and politely decline or counter-offer if you don't like what you hear.

Also- don't under-value your experience. It took you years to get to the level of expertise where you could write that dissertation, so you can help them avoid years of similar work. For example, in my field and location that would equate to a minimum of $120,000 a year for a fresh PhD grad, so a company would stand to save a hundreds of thousands of dollars by getting you to jump-start their project, plus they'd have the benefit of getting to market faster.

Ask a reputable friend or go to your advisor to figure out what a reasonable price for your first consulting gig might be. Specialized consultants (and your dissertation work would certainly be considered specialized) are routinely paid hundreds of dollars to upwards of a thousand dollars per hour. Be realistic, but also realize that you apparently have an in-demand skill set.