If it truly only has 12K on it (very hard to really tell, and looking over the "rest of the bike" will tell you LOTS more than the odometer will) I suspect he may be correct on the valve guide seals, but then, you're the one "placing the bet".
Assuming just seals would fix it, and that you can't install them yourself, you'll need to shop around in your area, as the cost to get it fixed could vary by a bunch - If you're needing a "number" to negotiate with, I'd go with $500 to $750. I suspect if he could have it repaired for that amount, he would have done it a while back, as it would have increased the value of what he's trying to sell by more than that . . . . .
If it was me (and I just did LOTS of shopping, and bought a CBX about 3 weeks ago) I'd make him an offer on the bike after
it's been repaired - If that's not possible, I'd plan on it being at least $500, and possibly a good bit more, and go from there.