If it's the fully marked version with NO 9 Griswold Dutch Oven Oval Roaster with NO problems with everything in black iron, I would think anything under $500 would be a good price. Probably $700 or so top dollar these days. I've seen them sell for up to $1000 and more but that was back eight or 10 years or so ago. OR's with the plain lid or just marked OVAL ROASTER would be worth less. The BB gives a value for the 9 trivet in black iron as $125 but I think a number more like 200 - 250 is more realistic. Remember just my estimate
There is a deep version of the 9 OR in black iron with pn 2675 on the oven and 2626 on the lid and IMO it is a rare, rare piece. I only know of one that has changed hands in all the time I have collected but I can't venture a guess on what one would go for other than I would expect a lot more than the standard fully marked 9, maybe a 50% premium? I can't recall for sure but it may even have it's own trivet with a different PN from the standard 9 trivet. Just not sure on that anymore, getting to old. If the dates for the standard #9 in the BB are correct then the deep OR was made for a longer period than the BB states. They should be around as they were made for at least seven and probably more like ten or so years, but I've never seen one so I hope that's what you end up with. Again, remember prices are just my guess and someone may come along and shoot these #'s down or pump them up.
Harry