It all depends. You can find minis from free (at least in initial cost) to thousands of dollars. Age, training, breeding, registration - it all makes a difference. Location matters, too. If you can find one locally you won't have to figure in cost of shipping to you. I'd suggest looking around locally and see what is available. I paid $1500 for one of mine, and $2500 for the other.
I have a free one that the previous owner decided to give me when I looked at her other horses. She is elderly and thin, but such a doll! I bought two of her little friends. Total for all three was $900.00. One is registered and the other two are considered grade. And one is pregnant too (not planned for). If you plan on getting one, try for two as horses are herd animals. Around Michigan the average pet quality mini is anywhere from about $300-$1200. Most seem to average around $500-$600. As above, there are some free ones, but they will likely cost more in feed, farrier, and vet bills. Just please make sure this isn't a bday present that will be tossed aside by a child when they grow tired of the chores. The adults will also have to take responsibility. My kids have all had their "own" pets for years, but sometimes they outgrow their desire for the pet, the chores are too boring, they stop spending time with the pet, etc. My kids were all in 4-H, which would be great for any child too, where they can have support and something in common with their peers. All of my kids pets are really mine in the end (they go off to college, etc) so that no animal is neglected over time. Not trying to be negative and I think any kid would be elated with a mini!! Just go in with open eyes. I am learning something new every day (thanks to this forum!) By the way, the one you have pictured is a beauty!!
Hello! In our area mini horse prices seem to average $300 - $500 for an unregistered mini with trained and registered going higher.
Does your niece, or her family, have experience with horses? Her experience may determine the best investment. It may, for example, be a better investment to pay more for a horse that has “been there done that”, if she is newer to horses. Good luck and have fun! Horses are a constant learning experience with never a dull moment!
In Colorado, unregistered ones are usually around 500-800 these days, 1000+ if trained to drive or leadline. Sometimes you can get super lucky and get one for 300. That is what my gelding cost 8 years ago and he is the best pony ever, but I did hit the jackpot with him.