I got my daughter a filly last fall she has been wanting one for years and been riding sence she was 5. They are not a lot of work but you still have to mess with them at least an hour a day. Just for fun this is what you can expect to spend.
300-800 for a horse
500-1200 for a registered quarter horse or paint
300-500 for a kid saddle (figure on buying a bigger one later)
100 bridle
25-50 to have its hoves trimed after it loses its baby hoves
25-50 to have it shoed
100-200 on hay a year
150 a year for grain
50-100 grass seed after then kill all the grass when the ground it soggy during spring.
50-100 fertilizer
40-60 for 2 halters if it is a colt when you get it less if it is full grown
150 odds and ends like lead ropes, lung lines, blankets, combs, brushes ect.
180 for DNA test if you want to register the horse as a quarter horse or paint
if you want to see her horse here it is
Horse