There is not much that I dislike more than trying on jeans. There can be 12 pairs of jeans in my dressing room, different brands but all the same tag size (my tag size), and not a dang one of them will fit correctly. One will gape at the waist, one is too short, one doesn't come up past my thighs, one goes up to my armpits, one is too tight, one is so big that I could fit another person in each leg, and so on. It is also guaranteed that if I do happen to find a pair of jeans that fit and are comfortable, that the manufacturer will immediately stop their production.
Fitting a saddle to a horse is just as frustrating and 30 times more expensive. Saddle fit is tremendously important for both rider and horse and a saddle that fits both is the Holy Grail of the equestrian world. There are people with unlimited budgets who can afford to have a saddle custom made that fits both parties equally well. And then there is us. The rest of us, most of us, have very limited budgets and must make due with off the rack models.
To imagine how a horse with a poorly fitting saddle feels, put on a pair of shoes that don't fit, fill a back pack with 20 pounds of sand and then jog around the block. Your feet will hurt, your back will hurt. Hips, knees, shoulders, all of 'em - aching. Now, do it again the next day.
Saddles do not have to be $5000 custom made saddles in order to fit correctly. An expensive saddle can fit as badly as a cheap saddle, if it's put on the wrong horse. So how do you know if a saddle fits? I can share what I've learned about fitting English saddles (excluding Saddleseat) by paying attention to saddle fit experts, reading and watching instructional videos.
First, examine the saddle. If you pull the pommel and cantle toward each other does the saddle bend in the middle (it should not)? Does the gullet (the channel underneath) go straight and evenly down the center of the saddle? Are the panels of even size and shape? Are the billet straps solid looking (no oval shaped holes, no splits, cracks or thin spots in the leather)? Is the padding in the panels smooth (no lumps)? When viewed from the front do the seat and cantle line up evenly (not twist to one side)? If the saddle passes inspection, it can be placed on the horse. You will have to check if it gives the withers at least 2 fingers height under the pommel, if the gullet gives clearance for the horses spine, if the middle of the seat is the lowest point, if the billet straps point straight down (not angled forward or back), if the panels distribute pressure evenly down the length of the saddle and whether the saddle rocks from side to side. If the saddle passes that test, you can fasten it on with a girth, which can then greatly affect the fit. If the saddle is still in contention, it is time to sit on it on the horse's back. If the saddle still sits level, gives wither clearance and clears the spine. Then it's time to ride. Knowing your own horse well, you should be able to tell if he is comfortable by the way he moves. Is he stepping out freely, willing to go forward and relaxed, or is he mincing, grinding his teeth, carrying himself crooked, reluctant to go forward or round, swishing his tail, kicking, bucking or rearing? If so, you should get off. Then try the next saddle.
All of the trials should be done sans saddle pads. Pads can alter the fit for the better or detrimentally. Sometimes saddles are built evenly and balanced, but horses are not. A horse that is built downhill or uphill may need a balancing pad to fit a saddle. A horse that is swaybacked or has tremendously high withers may need extra padding to level the saddle. If a saddle fits all other criteria but needs a little balancing, then an auxiliary pad can be used. No amount of padding can make a badly fitting saddle comfortable for the horse. Do lots of extra socks make your shoes fit nicely - so nicely that you could either dance a waltz or run the hurdles? It doesn't work that way.
The good news is (Sarcasm Alert: all gullible people should skip to the end), as soon as your horse either gets in shape, or goes out of work, the saddle doesn't fit anymore! And we start all over.
While you don't need to take out a loan to buy a saddle, getting the $150 deal (stirrups, girth, bridle and bit - all included!) isn't going to swing it. The saddle (stirrups, girth, bridle and bit) is cheap because it is made of cardboard and was sewn together by nervous hamsters. It isn't worth $150. Unless you want to pay $150 for a saddle shaped paperweight. Used saddles are certainly a good option, provided they pass all the pre-horse-wearing tests. The prices can be very reasonable and if it's good quality leather and construction, a saddle can last for decades. A good quality saddle will aslo have a good resale value should you run into the situation described above (which all the gullible people don't know about because they skipped ahead).
A saddle that fits well for both horse and rider will make your riding time more productive, more comfortable and more fun. It is worth the time and hassle, much like the hunt for the perfect pair of jeans.