Right now Cydia wisely will not let you purchase tweaks and apps on the 7.1.1 jailbreak. Although it should only be a few days until you are able to do so (kind of like when the 7.0 jailbreak was released-), I've come up with a workaround.
If you have a jailbroken older iDevice (like an older iPod Touch or unused iPhone) that is already jailbroken, purchase the app/tweak on it and then when it comes to downloading it to your new, 7.1.1 iDevice it'll have 'install' in the upper right corner, allowing you to then do so.
If you have a jailbroken older iDevice (like an older iPod Touch or unused iPhone) that is already jailbroken, purchase the app/tweak on it and then when it comes to downloading it to your new, 7.1.1 iDevice it'll have 'install' in the upper right corner, allowing you to then do so.