I trade iron condors on occasion. No, it doesn't mean I'm smart. It just means that its one way to try to make money in the markets. Like everybody who trades iron condors I'm well aware of what my worst case loss can be. As for any option trade sometimes they make money and sometimes they don't. There are iron butterflies, synthetically equivalent to regular put and call butterflies. So are they strong or ephemeral?
You have discovered the skew. Because most asset prices can go down faster than they go up OTM puts at a given distance from the spot price cost more than OTM calls at the same distance. The market is telling you that there is more risk in the short puts than in the short calls at the same distance from spot. The market is usually right, so in an IC consider selling bull put spreads further down than the bear call spreads are up, assuming you have no directional bias. I've never traded forex options but my guess is that this is a market that should be the exception that proves the rule -- I'd expect calls and puts equally far from spot to cost about the same. That's because of the symmetry of forex -- USD/EUR crashing down is the same as EUR/USD crashing up. Forex pairs crash up as often as they crash down. So if the skew irks you try IC on forex. As for whether the premiums are worth the risk -- that's a matter of your judgment of what the probability is of hitting the strikes. An IC that pays out $2000 while risking $20,000 is still a good play if the odds of staying within the short option strike prices is 95%. As with all option trades you won't make money unless you're better than most at judging how far prices will move in a given period, or what the volatility will be.
An iron butterfly is basically a degenerate iron condor in which the short call and put have the same strike price. So I fail to see how an iron butterfly can be "equivalent" to any iron condor other than itself.
Gold also: the skews on GLD and GDX are reasonably even, although slightly skewed negatively, at least last time I checked. OTOH, anyone trying an IC on these should have their head examined, IMO, but the skew is easier to deal with, anyway.
Skew is a fact of life in many options environments. Rale against it or exploit it, it's your choice. I do not put on an iron condor as one trade. Rather, I put it on as two credit spreads. The distance from the current price of the underlying is controlled by the probability of touching the short strike and the credit I can achieve. In some markets, the returns between the spreads can vary by 2:1 or more. On many occasions, considerable time passes before the second half of the IC is entered. I am just as stringent about my probability of touching rules but less so on the credit than on the first half. After all, the second half of an IC requires no additional margin.
I will simply say that most people don't really understand the skew. It's telling you about volatility, and while that is related to probability, it's related in the same way the risk free return is: vary the risk free return, and you vary your probabilities. Same with volatility. But probability is probability, period. I see a lot of fancy terms being thrown around about it, but it's best to keep it simple. In an IC, it's all about the chance of either coming close or actually getting touched on the strike. Because you have to think very hard about what to do once this happens: you're looking at very high gamma, and you don't know what the future holds. Will it continue through, or reverse? You don't know. And you have to know you don't know, and know you won't know when it actually happens. I only look at the probability of touch, and while I use TOS, which gives you this probability, the probability it gives you is based on a skewed normal distribution, which is not the real world. It's kinda sorta like the real world, but kinda sorta can get you murdered. So, I worked very hard, before I placed my first IC, on coming up with a realistic probability of touch, not based on anything normal or related to it, and on a strategy to follow once it actually happens. Which it has, this month, so now I'll know whether what I came up with actually works. If not, I'll try something else. I'm only devoting a small amount of money until I really come up with something that does work in this situation, and as I have no deadline other than what I impose on myself, I really don't care if it takes years before I come up with something that satisfies me. My researches showed me that people have come up with all kinds of different and sometimes truly weird ways of dealing with this. Obviously, the key to IC's is dealing with this correctly, and the only way to truly know which way is correct is to find out through experience. So, that's where I am.