Your very reasonable and logical question touches a pet peeve of mine, "... dies for the same caliber going from cheap to ridiculous. why ?"
Mostly, it's for the external finish! With at least one maker, a large part of the original cost is to cover free replacement parts for those who bust up their gear for life. I don't damage much so I'd rather pay for a couple of replacement things when I need them than to pay for them for everyone else, forever! With some exceptions, as noted below, the die interiors are effectively the same.
Purchase price, taken alone, is a poor guide for buying anything. We never get more than we pay for but we often pay for much more than we get! I've been loading for over 45 years and have learned a lot. One of the most important has been to buy tools for the design and not the cost, brand or reputation. That's because I've found less expensive tools, without exception, make ammo just as good as more expensive ones, on average, for a given level of user skill.
Many stauch believers in their beloved specific brand of dies would likely change their tune if they would buy and correctly use a cartridge concentricity gage! That thing can take the wind outta someone's puffed-up sails faster than anything else I've ever seen!
For the best ammo, we need the straightest cases possible. I've found that Lee's inexpensive Collet Neck Sizer die leaves necks the straightest of any sizer and, used with a good seater, it will make some very straight ammo.
I've read/heard many boasts of the customer service from a few brands but I wonder, first, why be such a clod as to need so many parts replacements and, second, does that mean the C.S. policies of the other makers are less reasonably supportive? I don't think so but far I've only had to use the services of one "famous" brand.
"what makes one manufactur's dies better or worse than the others ?"
Mostly, they really aren't. Brand reputations vary greatly tho. Mostly because some makers don't give free "samples" to mag writers or help them develop silly wildcats to write about.
"why are not all dies created equal ?"
Cost. It costs a lot to polish the non-funcitonal exteriors of dies and it gives no benefit.
Those few dies that are, indeed, better, with more mechanically complex designs - Redding and Forster for instance - are only a little better, on average, than more convenitonal but less expensive dies. But, it really does cost significantly more to produce those premium fitted chamber type seaters w/micrometer heads.
"do they not make the same end result ?"
Pretty much. Much noise is made of some brand being made to "tighter tolerances" but I question that. SAAMI has a tolerance range, max to min., and I think all makers stay within those tolerances. So I wonder if the "best" makers are supposed to stay "tighter" towards the max or tighter toward the min tolerance, and does that really matter anyway. In fact, for conventional dies (excepting Redding/Forster Competion/BR dies) I've found that the makers variations will vary as much between their own dies as between brands. Bottom line, the most critical factor in making good ammo with any die set lies mostly with how good you are and how well the dies match your firearm's chamber. And that fit is a turkey shoot! I once made a chamber cast of my .243 and then bought six sets of conventonal (RCBS) .243 dies before I got a mixed set of a sizer and a seater that reasonably matched my rifle! (Sold the others, ebay bid + shipping will often fetch more than retail prices!)
The Redding/Forster Comp seaters do almost as well as the excellant Wilson hand dies and do it with less effort. Together with a good sizer, those seaters make assembliing straight ammo pretty easy. That is, after the case necks are turned just a little!
I have found little or no average improvement using any other brand's seaters, including those with some sort of short sliding sleeve gimmic such as the Hornady ND dies, the old Lyman, Herter's or Vickerman dies or current RCBS Competition dies. They can make it a little easier to get short bullets started but at increased cost, sometimes at a greatly increased cost. In fact, I find the inexpensive Lee "dead length" seaters do as well as any conventional seater IF the bullet guide section is snugly fitted!
Forster's sizer dies have a very good neck expander button that can be adjusted high on the spindle. It can start expanding immediately as the neck is being withdrawn from the die's neck, that helps keep necks straighter when FL sizing. Or, just use a Lyman "M" expander after FL sizing to help keep necks straighter. The "M" dies are made for cast bullets but they work great for jacketed bullets too. "M" dies don't have a decapper so they require using a universal decapper.
The point of all this is to justify my final statement; Making the best ammo can require finding and using specific dies from different makers, not just blindly using what each maker puts in their boxes, reqardless of cost.
