Well first of all CONGRATS on the fact you will be getting a horse. That is so exciting and such an amazing feeling!
So, you do a lot of different disciplines, which is awesome. But, you probably don't want to buy a dressage saddle and a separate English saddle with a more shallow seat for your jumping and English pleasure. If you want to start out with one basic saddle, I would suggest an all-purpose saddle. I have an all-purpose and I really enjoy it. It gives me the comfort and security that I need at my level of riding, as well as the versatility to be able to participate in different activities when necessary.
Different saddles are different prices for... different reasons. Hahah. The leather could be different, the internal parts of the saddle may be different, it could offer features that some other saddles don't offer... but don't be fooled. As you get more experienced with purchasing saddles, you will realize that new saddles don't always mean the best saddles. I purchased a saddle that would retail new for probably close to $2000 or at least over $1000. I got it used for around $250. Remember, saddles are meant to get worn in, comfortable, and this means they will get scuffed up and have a "used" appearance -- especially if you get leather. Used saddles can be absolutely perfect, but it might take a little bit of time to look around for them. The way I look at it (and if you are looking for uses you can use this advice as well): look at as many saddles (or horses) you possibly can because when you find one you like, looking at all of the extras only makes you more certain you love the one you do... if that makes sense to you. (:
New saddles can also be great. Through companies like Dover Saddlery, you will have the option of ordering a demo saddle, but you have to actually pay the saddle's price first (you get the full refund if you don't want the saddle anymore). Dover Saddlery also sells saddles that are used or flawed if you look in their catalogs, but if you look at some local tack shops, you will probably have more luck finding more inexpensive options. I'm not sure what your budget is, but typically Dover Saddlery's good quality saddles are more expensive since their main saddle sales are probably the newer saddles.
Be careful when you are purchasing saddles -- stay SMART. It can be difficult if you purchase a saddle that doesn't fit your horse or if you purchase a saddle that doesn't fit your discipline (which is why an all-purpose saddle is a great idea for a safe saddle). Websites like Stateline Tack and Chicks Saddlery may offer saddle packages for cheap, but they will not be the same quality as some comfortably worn and used saddles. Of course, if you can afford the saddle package (includes the whole set of tack) and not much more than that, it is a good option as a starter saddle while you save for a higher-quality tack set. However, I would highly recommend purchasing a saddle that is a better brand because it will hold up better, the leather (or material) will be better quality most likely be better... I mean, if you look at cheap saddles and then look at the better quality saddles, it's a no-brainer which one is better. However, like I said, if you are on a tight budget the cheap saddle packages might be perfect for you.
My favorite saddle brand is Stubben. The new saddles retail pretty pricey in my opinion, not the literally most expensive saddles I've heard of as Stubben is a pretty well-known saddle brand, but they are expensive. Like I said, find a good used Stubben and they can be wonderful. I also love love lloooooove Marcel Toulouse saddles, they are so soft and comfortable, and absolutely beautiful saddles. Wintec saddles are less expensive and they are synthetic, so they are less maintenance. Although I have a leather saddle, it really isn't that much maintenance. I mean, I could stand to clean my saddle more, but I would rather have to put in some maintenance and have the saddle I really want than just not clean a saddle that I don't like that much... hopefully that makes sense.
I would highly recommend having your horse (once you get it of course) fit for a saddle so you know your size when you go to purchase saddles. Some saddles are higher cut in the wither area, some are wide, narrow, regular, the horse may require a special pad... saddle fitting is complicated and has a lot of possibilities. There are some things that a saddle fitter (or horse chiropractor, which I have never used) can tell you that many other educated horsepeople may not recognize. I cannot stress enough that you need to make sure you purchase a saddle that fits your horse. Even if a great deal comes along, such as a Stubben for $125, if the saddle won't fit your horse, it isn't worth it. Aside from making your horse uncomfortable, a bad-fitting saddle is simply unhealthy, and can actually impact your performance on your horse, as well. Make sure the saddle does not sit forward or backward, and that you have it placed on the proper part of the horse's back (so I used to take riding lessons and I was putting the saddle too far up on my horse's back, even though I had been tacking up for years on and off). You can find these routines online, but of course they won't compare to having someone experienced actually check out the horse and saddle.
Make sure that when you purchase your saddle, you are aware of the refund/trial policy. You can find plenty of saddle options on Craigslist or other used-saddle sites, but I would really recommend going to more tangible stores and making sure you try a saddle on your horse. You should have a more experienced person, aware of the saddle-fitting routine, examine the saddle during your ride. Saddle-fitters are amazing amazing amazing for this. They can do more than just analyze fit... they can examine horse conformation, movement during riding, etc.
Good luck with your saddle... but more so your HORSE!! (:
God bless, englishaqh (: