That's generally how people do rules as written, as it saves a good deal of book-keeping. If you find this answer unsatisfactory or if you fear your players might resort to "Hex scumming" whereby they assess the value of the land to get a 9gp value before expanding out their personal domain, you can do it more manually. Before you decide to add this level of work, though, ask yourself if this is a problem you expect to encounter. It's a good bit of work to clear out a hex, and leaving it unsettled means it could devolve back into being infested with lairs. Worse, the odds of getting 9gp is 1/27, meaning they might well have cleared enough hexes to build almost two domains if they're of average luck. If you let them have their carefully located 9gp domain, it will be at the expense of the months spent clearing while not also gaining domain income.
Ok, with that spiel out of the way, certainly you should feel free to be more specific. This is fairly simple: roll the value of the newly secured hex being integrated into a domain, and add it to the product of the current land value of the domain and the number of hexes previously owned, then divide that total by the new number of hexes. This will cause most domains to tend towards the average of 3d3, with slightly more favorable numbers for the powerful ruler who gets to scoop up the juiciest hexes adjacent to their personal domain before leaving the scraps to their vassals. God help you if you decide to allow non-contiguous personal domains, which has plenty of historical precedents but will result in the kind of bordergore travesty normally reserved for people who play Crusader Kings II.