Hi!! I hope you don't mind me adding my thoughts.
Whatever breed you choose, good chance is that some of your readers will google it just to see what it does look like. So you need a horse that looks like "WOW" to you and whatever non-horse person. You received good suggestions with Andalusian and Friesians, who are always a "safe choice" whenever you want something flashy. I would avoid warmbloods like Hannoverian and such, who can be 100.000$ worth of grand prix dressage horses, but if a random kid googles them, he/she'll see just a "brown horse" : On that, when you decide the breed, know that not all breeds come in all colors, many breeds lack the superflashy, super cool colors, or the flashy paint patterns. Like, you cannot have a buckskin tobiano arabian and have him be purebred, or a night black friesian with white socks and a nice star on the head.
Now if you want the horse-breed to be an "experiment", you need something a bit more exotic. I don't know how common they are (not very, I suspect), but I think you might have a look at the Akhal Tekč breed (it's spelt in many different ways so check that too) http://weirdanimalreport.com/sites/d...?itok=s1R7hzK1
It's the kind of horse that you see once in a book and remember for life, because they are actually shiny
. You could make a great description of a golden horse literally shining in the field. They are very rustic horses, used to harsh enviroments, so it could be realistic that one survived and did well alone.
Honestly, way more realistic that one of the horses that are "awesome" for their mane, would have the mane survive rain, mud, plant seeds and the such and still look awesome. Long fluffy mane after a few years outside could become no mane at all, uneven mane with bits missing, very short mane, mane so tangled with plants that it has a little Tarzan hanging from it... stuff like that. And, if you make the horse black: No black stays truly black in the sun. If always fades a little to reddish brown, and it's not particularly pretty. True white is almost impossible to find on a horse, and "reasonably white" never
means "reasonably clean".
Whatever you choose, remember to place the girl-horse scene as far as possible from a rainy day. Wild horses can and do look beautiful and reasonably clean, if it's been dry. After a rain, most horses look like this: http://www.barrowfarmrda.org.uk/IMG_3213.jpg
The price is a bit complicated. Some breeds are either exotic enough or fashionable enough to be worth "something" just for being of that breed, but I don't think that you could go over 10.000 (probably less, but again that depends on the breed)
The truly pricey horses are like that because they have both a stellar pedigree and they won an impressive numbers of competitions. It is quite unlikely that an exotic, flashy and "experiment" horse did something that amazing: the common breeds are common exactly because it's likely that they will do well in competitions. Exceptions do exist, but they are exceptions. Also, a thing I always HATED
about most movies is the equation: "horse is fluffy and lovely = dumb kid will win the race, no matter the breed or soundness of the horse, against all odds". So please, please, make this horse awesome, but if could avoid making him and the girl win some unlikely race.