To build on Mickeyluv'r's great post (if I may be so bold)...
The advice regarding shopping around is important. You may feel like every store has all the same stuff as many stores don't carry the 'unique' merchandise they used to, but you can often times find better variety or different price scales.
For example, one great souvineer that actually is useful is the beach towels you can get at different parks. They are a great souvineer because it's something you actually use, and they are really pretty. There are some that are as little as $18, and some as much as $30, depending on what park is on it or what design. I've actually found some of the cheaper designs to be the prettier ones! So the best advice is really to shop without buying for the begining of the trip, and get a feel before you make any rash purchases.
World Showcase, as mentioned, is also a great place for unique bargains. Besides the smaller stuff available in Japan and Mexico (small wooden toys, boxes, beads, etc.) there is also a lovely shop in Italy that sells beautiful Italian marble (some faux, some not) candle holders and such that start as little as $8 or $10. Those make great "mom", "aunt", or "grandma" gifts and I can't believe how cheap they still are (they've had them for many years).
I find the prices of clothing at WDW to be a bit much for me. Some of it's not bad (T-shirts can be had at a reasonable cost), but $30 for a baseball cap has always seemed a bit much to me. I've never found one I liked enough to shell that much out for, even though I always look.
I generally buy one nice thing (like my Cinderella Castle "toy", the best $65 I ever spent - it lives in my living room and I love looking at it every single day) or several pins during my visit. I don't get caught up in trading or really even collecting them, I buy them as mementos of certain days or times and each one I end up buying has a reason or story behind it, LOL.
The best thing is to set a realistic budget as to what you can afford, and go from there. I often times find it nice to over plan a little bit for spending money, and then divide it up by day. Each day I give myself a budget of X-dollars, and if I don't use it all, it rolls over to the next day. I always have money left over on the last day for a treat or two.
I don't really get caught up in shopping, though, and I know some people do. I'm usually only on the lookout for gifts for those at home or something meaningful that I can't get elsewhere. I've noticed that, especially in the last few years, there are a lot more items at WDW in the sub $10-12 category that are really more than "junk" one might expect to get for that price. For instance, some LOVELY little snowglobes (palm-sized) in the Emporium for $11, or some really cute little palm-sized plushes for $6.
Good luck!
NED