Bing's Ads platform isn't bad. It's definitely not as refined as Google's, but the big-G's spent probably four orders of magnitude more engineer-hours on AdWords, so that's to be expected. Bing gets the job done, though--it's like a major league version of a minimum viable product.
What's weird is that I often hear claims that Bing is "cheaper", or Google is "more expensive", and these statements are made in the ambiguous context of "sure, you can use Google, if you want to spend 30% more on your ads, hurr hurr hurr". They all seem to use the number "30%" either way, which is arithmetically frustrating at best.
So, what's wrong with claiming that Bing is 30% cheaper than AdWords?
First, you're not paying for ads, you're paying for clicks. If you spend 30% more on those clicks, that's your fault. You set your bids to whatever you want--it'll just dictate the volume you can attain at that bid level. So, if you're going to add a minimum number of clicks to the claim, maybe--but no one does, and this results in a huge amount of confusion around what's actually being said.
I'll agree that competition is lower on Bing. The tools are physically painful to use, if you're spoiled by the relatively mostly-okay AdWords interface, so even if someone is familiar with Bing, they might not opt to use it simply because it takes so much more force of will. Or, if they do use it, they're not tending it as closely. It's painful, so you're not leaping at the chance to go through that pain for a considerably smaller audience. The result is you go up against fewer competing bids in each auction.
It's entirely true that reduced competition necessarily produces a different supply and demand curve for Bing Ads, compared to AdWords...
Why, then, am I hating on the "Bing is cheaper" claims?
It's pretty straightforward, actually. If your Bing campaigns are producing a 30% better ROI than your Adwords ones, then it's a symptom of ineffective bid management, more likely than not. If your campaigns were running so lean, why aren't you turning up the bids, capturing more market share, and making more money? (I am assuming that your AdWords campaigns are hitting your target ROI, based on your margin expectations, though--if I'm wrong about that, then why aren't you turning Google down?)
All things being equal, in terms of attribution...
The ROI of two PPC search channels, managed diligently, should almost always be equal--with the differences in competition mainly shifting your relative market share within each ecosystem.
There's one exception, though, worth noting--bid saturation. If your dominance in Bing over your competitors is greater than your dominance over them in AdWords, then you may reach the point where even a bid of $5.00 only pays $1.80 per click, if the auctions pan out that way. I'm of the (very subjective) opinion that Bing's broad matches are a bit more liberal than Google's, so you may end up paying for fringe terms that don't perform as well, diluting your numbers back down, but the fact is that you could simply "win" that niche of searches. Once you've done that, sure, Bing is "cheaper".
That's not the case with most accounts, however.
If you're paying more (as a percent) for AdWords revenue than Bing revenue, you should very carefully consider the bid management you're applying to your AdWords account. At least, you may want to simply update your bids more often, or ensure that your bid decisions are being made based on statistically significant observations.
Should you be on Bing? Probably! You should be on Bing because there are shoppers there that you're ignoring otherwise. You can own more of the Bing market than you can the Google market, but it's more labor intensive, so plan your allocation of energy accordingly. My (well-founded) fear is that people see the claims about it Bing being "cheaper", use Bing's import tools to copy their AdWords campaigns over, and then forget about it for a few weeks--and lose a mint.
Remember, a smaller slice of Google's market is still bigger than a big slice of Bing's.