If you've never trained a baby before add the cost of a trainer to help you with the baby.
To answer your question I looked at that general criteria. Least expensive was $1000 for an 8 yo green broken WB mare who hadn't been worked in iover 2 years. Appears it may be outside the US but US dollars was quoted.
Then I saw a 4 yo reg WB mare for $10,000 - mare currently being ridden, VERY nice mare.
Things that decrease the price - if she is not currently being ridden price goes down because you need to treat her like she's never had a rider on her back. (At least I would not be the first person on her back after 2 or more years). It also makes you question why she didn't have more under saddle work on her at age 6 - was there a soundness issue?
Also - where are you located? That makes a difference. For example: Horses are more expensive in Cal. Than in Michigan or Minnesota.
What is mare's height? Less than 16 hands decreases the price (generally) and more than 17 hands also decreases the price (generally).
What discipline is the mare most suited for - and does it match what you ride? For example if mare moves very flat then she's good for a hunter (as long as she jumps well) - so if you want a dressage horse you would probably choose another horse. What is temperment like? Her foals temperment?
Personally I might offer $5000 for her IF she had solid ground manners and the vet's physical came back (xrays, etc.) with no issues. I probably would NOT count on breeding her to the stallion but do have them list it in the contract (if you buy) with an expiration date or mentioning that there is no expiration date and what happens if that stallion is sold before you beed to him.
Dressage is for Trainers!