Ok so this credit card gets you this many miles, that credit card gets you that many miles...I have no idea what all of this means. 
I'm sure it varies by card, and what time of year you are flying, etc.
Here's the situation...
We pretty much only fly places southwest flies, BUT we want to go to Hawaii. We have no qualms about opening a credit card and then closing it JUST to get the free miles or whatever as we have an emergency fund and no plans to open any new credit lines, so we don't mind taking a hit to our credit line for this. We don't make a habit of doing this, but would do it once for a free or cheap flight to Hawaii. So for example, I think I've seen a 15,000 mile offer on Delta's card lately. Can someone tell me what that MIGHT be good for? Probably not enough for a flight- maybe good enough for an upgrade?

I'm sure it varies by card, and what time of year you are flying, etc.
Here's the situation...
We pretty much only fly places southwest flies, BUT we want to go to Hawaii. We have no qualms about opening a credit card and then closing it JUST to get the free miles or whatever as we have an emergency fund and no plans to open any new credit lines, so we don't mind taking a hit to our credit line for this. We don't make a habit of doing this, but would do it once for a free or cheap flight to Hawaii. So for example, I think I've seen a 15,000 mile offer on Delta's card lately. Can someone tell me what that MIGHT be good for? Probably not enough for a flight- maybe good enough for an upgrade?