it's very hard to tell you how much your horse is worth without knowing the market you are selling in. You can figure out the best price yourself by looking around at horses for sale in your area. Look for horses that have comparable training and breeding and base your price off of that. Also, you never mentioned water... is the horse scared of water or baths? Grullo is often seen as a more "rare" color, so if you do sell her then it might be easier to find a buyer because of it.
Where i live horses are dirt cheap and you might get 1,000 if you are lucky. If she has OUTSTANDING bloodlines and was trained by a FAMOUS trainer, then maybe a bit more. I used to live in California, where i'm estimating she would have sold for anything from 4 to 6,000. The region where you are really makes a big difference for how much your horse is worth.
If you want my opinion i would keep her and train her more. If you still want to sell her then you can raise the price due to training, but if she turns out nice you'll still have a nice horse. Good Luck, and remember to look around your own horse market!