Well you can purchase it. (Sorry couldn't resist the dumb answer)
I would first backup your device, then do the jailbreak. There should be no reason for the online banking app not to work.
Use Evasi0n 1.07 for jailbreaking and you should be good to go.
If not you can always reload the current IOS and be no worse the wear.
Typically I would suggest a fresh IOS install then jailbreak followed by a restore of your apps, but current is now 7.1.1 which at the moment has no working JailBreak.