The horse may be in a bad situation but unless you can improve the situation and afford to keep the mare in better standards I would see no point to go any further.
A 4 year old cross....but what does she know and what kind of health issues are present would dictate the price offered.
Is she trained to ride, does she drive?
How far advanced is her training and can you further it, better it or at the minimum maintain it?
Is her health compromised or is she yet healthy physically and mentally?
So much of what she is worth is also going to be from her current owners because they know you want her so they can ask a price and you either go yes or no and negotiate.
A cross of Morgan and Freisian is not unheard of...but is she from a cross that has some kind of records in their past of achievement? Either parent and further back in her lineage?
How large is she, how does she herself move, is she athletic and have that presence that is what catches so many eyes?
So many things to consider...
With that though is also the fact that she is only worth meat price as so many gifted and well-trained young, sound horses are turned away and sold, ending up on the auction block and then a hip number some rescue begs a stranger to pay bail on...
If she were to go to auction she is worth what she weighs... many horses go through bringing in only a few hundred dollars at most...it is kill-buyers who then put a ransom price over double what they paid to get the horse from horsey-jail....but, this horse, any horse is only worth what the market is willing to pay...
To me, depending on all those above mentioned....but is she worth much, sadly no.
Unless she is saddle-trained or broke to drive already she is not worth more than a few hundred dollars in honesty...what the market will bear and pay is key.