Hello!
Buying and selling horses is like buying and selling art work or craft work - there just really isn't a universal rule of thumb on pricing.
Here in my area of MN, if the mare were a safe and experienced driving mare, I think she would be priced somewhere between $1000 - $2500. She could easily go higher if she had a show record and papers.
My experienced driving mares were all older (around 8) when they began their driving careers in the hands of a professional trainer who did absolutely wonderful work. Training - way back then - was about $400 a month per mini (I'd have to look, but that sounds right). Training for each mini was two months, maybe three. Again, it has been awhile so some of the details escape me.
I do know that they are as sound as a dollar, mentally and physically and as trustworthy as possible for a living breathing animal with a mind of her own! They started out as sensible creatures and were in the hands of an experienced and thoughtful horse trainer who understands the value of time.
These things have kept the bit of driving I do (and that the grandkids do) safe and fun.
If you have an opportunity, I would watch her being hitched up, driven, and unhitched. I would want lessons with her owner on how to cue her, what are her 'quirks', if any? Has she been driven on the road, arena, state park, parade? How does she stand, lead, interact with people/dogs/horses? Load in a trailer?
You are surprisingly vulnerable in your cart even pulled by a mini. You can't ask too many questions!
Good luck and be safe!