Bing is cheaper because their reach is less. Bing+yahoo has about 10% of the search market, while google has 90%. I know that some people aruge that it's more 30:70, but beleive me I've done plenty of PPC campaigns and have a couple of high traffic websites to gauge the actual traffic referrals with.
Bing can be cheaper because of lower competition, adwords is the no1 go to place, where is bing and yahoo tend to be afterthoughts.
I agree with you adwords can be OTT expensive. There are some retailers who are willing to bid up to and beyond the margins for a product. This is paticularly the case with cosmetics because they know that they can upsell and cross sell other products.
I worked with a footcream retailer who spend £4 per conversion on a low cost £9.99 foot cream. It's ridiculous that the margins are so high on this, but there are other high street retailers like boots, pharmacy2u and other chains who can bid up the cost price really high because although they might only advertise a foot cream, in actual fact the basket purchase tends to be a lot more. So customer/visitor might end up buying other related products and making impulse purchases, if only just to get the minimum amount to qualify for free p&p.
So one top I would say is improve conversation rates, captures leads in order to retarget the same consumer with follow up emails.
Also adwords requires skill, you need to be experienced with keyword research, campaign setup and performance optimisation. There's a whole minefield of information about this that I won't get into.