I've never considered the $ aspect of it; but I can see how it would be very erotic to share your SO with another couple. If it's OK with both members, I don't see what's the big deal; and in fact, it's quite common, tho not nearly as common as cheating. So why not both fool around so that there's no cheating or dishonesty involved? And if it's together with another couple or group, all the better! I've had a lot of relationships that probably would have lasted a lot longer if we'd both been screwing other people.
Also, it's a common male fantasy to want to see your SO be fucked by some other guy, tho few guys have the courage to see it through - tho I bet a lot of wives would welcome the idea!
But as for having the wife screw other guys for $: it all depends on whether she was in to it, and if the relationship could take the emotional strain; when it comes to sex and "my woman", guys are not completely sane, and living off the wife's wages would be difficult for almost any guy.
This is one of those things that comes down to your personal moral code; when it comes to sex, it's definitely not a one-size-fits-all world.