Would a filly cost more?

Nope!

Young horses can be very expensive, and you will eventually need all the tack anyway. Much of the regular tack and training equipment are the same, and much of the training equipment is what you will need for a trained horse too, like longing equipment.

A young horse will change, especially in the shape of the back. A saddle that fits a horse as a three year old may not fit it as a 7 year old. A very important piece of "training equiptment" that many people don’t plan for is a professional trainer. Unless you have been riding for many years, not just weekend riding, but 5 or 7 times a week, with young horse experience, you should probably have a professional trainer coach you.

Your question reveals your limited experience. Less experienced horseman buy a lot more equipment because they don’t yet know what they need or how to select fit, suitability and quality.

If costs concern you, you should try leasing a horse. By leasing horses, you gain the experience to deal with, or get along with, a wider variety of horses and save money for the eventual purchase of your horse. Usually you get to use the equipment that fits the horse. With that experience and knowledge, and the connections in the local horse scene, you will have a much easier time finding a horse that suits you.

Wear a helmet. They are much more attractive than wheelchairs. If you think your hair is more important than your brain, you are probably right.
References :

As you’re asking this question, I’m going to guess you’re a novice rider. If that’s the case, than if you bought a green horse, you would have to pay a professional trainer to bring the horse along for at least several months before it would be safe for you to ride. Training fees can be quite steep, and I’ve never seen any that are less than five hundred a month, usually on top of whatever the cost of board.

No matter what, you will need all your own tack and equipment in the end. I recommend first taking riding lessons for a year or two. Also try to volunteer around the stable so you can see what it’s like to work around horses. When you own your own horse, you will eventually have to muck a stall, wrap legs, fill water buckets, and more. Then, when you’ve been riding for some time and you’re comfortable working around horses, I recommend leasing or half-leasing a well-broke horse before you consider buying one of your own. Most people who rush into buying without spending much time learning to ride and learning proper horsemanship find themselves completely overwhelmed by the amount of work and time (not to mention money) it takes to own a horse. Leasing provides you with a good idea of what it’s like to be responsible for your own horse. And if you decide it’s too much, the lease is only temporary so you won’t be stuck with an investment that cost you several thousand dollars. You can give it back to the owner.

Only when you’re financially and mentally capable of caring for a horse should you look into purchasing one of your own. And when that time comes, you can evaluate your skills and your goals to determine how much training your future horse should already have. I highly recommend always bringing a professional with you to look at prospects, as they might spot flaws in the animal’s conformation or behavior that you could miss. Also, vet checking the horse is absolutely mandatory. I’ve seen too many animals purchased supposedly sound only to break down a week or so later.
References :