First, the '9 letters, two words' means nothing; the domain could be AutoParts.com, worth $ a lot, or it could be PorkRinds.com, worth $ very little, or anything between. So that part is irrelevant to any kind of valuation, without us knowing the name.
Second, as mentioned above, a $10K opening offer could mean anything. Could be all that the company can (or will) budget for the domain, and they came in serious with their top offer. Or it could be a drop in the bucket and the company would pay $2 million, but of course the broker wants to start off as small as possible - but with an offer large enough to show you they're serious, rather than, you know, "Hey, we'll pay you $200 for this!" However, **usually** the opening offer is not the top offer. And an opening offer of 10K can often mean they are willing to go much, much higher. Again, just no way to know, without further details.
Way too many variables and unknowns for us to give you any real answer. The thing to remember is: you don't have to give them any information! Since you are actively using the domain for your ongoing concerns/projects, you don't have to come up with a number. You can simply reply:
"I'm not interested in selling this domain, so I won't try to think up a price. Thank you for your interest."
That is a great response. It's honest. It thanks them. It opens a door for them: if they are REALLY interested in your domain, they won't let it go at that. They will keep hammering at you to get a price from you, or keep raising their offer if they can, etc. So with that simple reply, you will see just how interested they are in your domain. Then the REAL negotiation can begin.
We understand you do not want to put your domain up for valuations in the appraisal section here and have it be indexed. If you want more input, you might cloak the domain (like an image/screenshot of it) and put that in this thread, we can give you better advice. Another avenue is: there are a few old-timers I see in this thread, most of them (and me) are open to you DMing us with the name and we can give you private valuations and advice and we won't mention the domain publicly.
Whatever you decide to do... Good luck! Always nice to know there's a sizeable company out there who wants your domain and brings a strong first offer to the table. Even if you don't really want to sell... well, if the price goes way, way up, maybe you will want to sell