Spaying usually involves removing the uterous AND the overies, whereas the much more equine-common overectomy just removes the overies. It can cost $400-2000 depending HOW its done.
First, the reason this is rarely done in the equine world except for specific reasons (tease/jump mares, various cancers, etc) for a few reasons. Most importantly, its major abdominal surgery.
The most common way to ovex a mare is to lay her down on an opperating table and open her abdomen just like colic surgery. This involves anestesia, risk of infection, danger of recovery, etc. It also takes a few weeks to heal. This is a very involved surgery which can cost upwards of $1500.
The least common method is to enter the mare vaginally with a tool which basically pulls the overies off. This method is far less risky, heals much quicker, and is done standing. Unfortinately, most minis are too small to do this way. Plus, if anything goes wrong in the surgery it usually results in euthanasia. This method is FAR cheaper, sometimes as little as $200.