Price will vary depending on the area. Here is a basic list of expenses
Hay for the winter
Grain (depending on the amount of work the horse is in)
Farrier (either trimming or shoes depending on the horse)
Grooming supplies
Saddle, bridle, girth, saddle pad(s)
Dewormer
Vaccines
Floating teeth
Fly mask, fly spray
Blanket (only if you live in the Northern US and/or your horse has a fine coat)