The most typical monsters with DR/magic are outsiders of various sorts: fiends, celestials, and elementals spring immediately to mind.
A few of the more potent undead do have DR/magic; vampires have DR 10/silver & magic; nightshades have DR/magic & silver, too.
In general, I’ve noticed a tendency for many monsters with high CR’s to have DR/magic…because in “normal” (by the book) D&D, magic weapons are the norm for higher level PC’s anyways, so it really only affects lower-level followers, minions, and extras. These creatures have this DR so they can wade through hordes of such “lesser beings” and basically ignore them - meaning the heroes must be the ones to deal with them.
One thing 3.5 did in regard to magic items though is that now any spellcaster can make them assuming they take the appropriate feats, so rangers, paladins and bards are in the pool. Let us not forget the magicain - a very likely magic item creator, especially for divination items, IMO.
True, but this was a 2e/3e change, not a BR-specific one. Correct me if I’m wrong, but I don’t remember magic items being any more difficult or expensive to make in BR than they were in any other D&D setting. The major difference was the rarity of true mages, not the multiplied expense of item creation.
In the BR setting, magicians will definitely be major producers of lesser magic items, and of more powerful illusion, divination, and some enchantment items. I also require alchemical production to require a touch of arcane magic IMC, a specialty of magicians as producers of special goods.
Rangers and paladins, well - if they really want to waste a precious feat slot to make what pitifully few magic items they’re capable of crafting, let the fools do it. 
Not even close to worth it IMO.
Similarly, bards and sorcerers are also poor artificers because of their limited knowledge of spells - it makes most item creation feats a waste for them. The possible exception is Craft Magic Arms and Armor, since it doesn’t take any spell knowledge at all to make them with basic enhancement bonuses. This is a flaw in the system, IMO - I think it should require a Magic Weapon spell to make +1 weapons, Greater Magic Weapon to make +2-5 weapons, and Magic Vestment to make magical shields and armor. I am assuming here, though, that mages would gain access to the Magic Vestment spell if there were such a requirement.
Wizards, of course, are probably the best all-around artificers, with clerics a close 2nd, and magicians 3rd. Mainly because they can take a single feat to gain the ability to craft a wide array of different items. All it takes is the proper caster level and spell knowledge to do so.
In BR, I expect the most common magic items are crafted with clerical magic, and low-power items of magician-make should be right behind them in frequency. More powerful items crafted by wizards are the ones that will be the rarest.
Now, in anything like a market economy, what that would mean is that the actual market value (but not the creation cost) of any item that can be crafted only with true magic should be vastly inflated due to scarcity - elven realms being the exception, of course (normal market values would work just fine there).
I have always been slightly uncomfortable with the idea that market price determines creation costs. I actually think it’s an OK system so long as the market values of magic items in the DMG or elsewhere are understood to be an absolute market value, one not adjusted for inflation, scarcity, etc. Item creation costs shouldn’t fluctuate unless it really is actually more difficult, draining, and expensive to make an item in that campaign setting than in the default [Greyhawk/FR] D&D setting.
I think making magic items more expensive in terms of market value and creation costs will have the main effect not of making them so much rarer, but of lowering their average level of power due to the exponential cost increase of higher-powered items.
Personally, I think lesser items are kind-of cheap in 3.5, but more potent items are plenty expensive - so much so that the typical pC or NPC will only make 1 or 2 items of great power in their careers thanks to the xp drain.
As far as greater profits go, well…so what? Sometimes it matters, but more often than not it’s the XP cost that is the prohibitive factor (for PC’s especially), not the gp cost or potential profit. For regents this is even more true, as money tends to be less of an issue for them than for non-regents.
Doubling or tripling the xp costs is a sure way to put a heavy damper on PC item creation. It’s also extremely harsh, and will tend to create even larger level gaps between spellcasters and non-spellcasters…which is OK if you want your PC spellcasters to be a level or three behind the other PC’s, bad if you want the whole party to stay abreast of one another in character level.