The best thing to do is to put some more value on her. Get the registration done now, continue training her, and good, flattering pictures/ videos are a must. Get good conformation shots and a few nice artistic-type shots, maybe of her profile, of her moving, etc.
If you show, try to trailer her out to shows with you- it's good advertising. Even if all you do is walk her around, it's good enough.
As for her price, it's what people are willing to pay for her. I can't say what I would give you, since I don't know anything about her, but for a greenbroke 4 year old who isn't fully registered, I wouldn't pay much. Horses like that are going for a couple hundred dollars where I live, or being given away.
Are you in a hurry to sell her, or do you just want to find her a better home and are willing to wait in order to find it?