The app developer has the option (and responsibility) to free up resources when the app isn't running in the foreground. In your example, lets say the mortgage calc uses gps data to find rates and prices based on where you are. When you back out of the app, it SHOULD :
Free the GPS sensor
Halt calculations to free the processor
Stop drawing and refreshing itself (you're not looking at it anymore!)
And basically sit idle.
It would sit active in memory, but not be using any resources. If you fire up the browser, and the OS says -"hey, I could use a bit more memory so I'm going to close you completely. Nobody will miss you because you haven't been used in a while. Now shut yourself off gracefully for me please." the app should exit completely and free the memory for other uses. Android does this very well. Most apps do this very well. When you come across one that doesn't, you'll see the users screaming bloody murder at the dev to fix it. Usually they do
When an app doesn't close up nice and tidy, Android can use a Linux command (kill
pid) to forcefully close it. There are numerous apps that provide a GUI to do this easily, but for the most part they aren't needed.
As for when any new devices are coming out...it's anybody's guess. Rumors say 10-15 more Android devices before April of next year, and I'm sure some of those will show up at Sprint. Maybe, just maybe, one of those will be the "Google Phone" that's supposed to be the game changing device we all want.
If it were me, I would consider waiting until the end of January (that +30 days means you're safe until almost March lol) but no longer. Trying to beat the cycle of new devices is a losing battle!