There are three types of protection you should consider in your case (four if you consider secrecy, but keeping a gui secret ... well... lets say three):
Since 1989 you do not have to register your copyright, you get it automatically. Registering it might help with protecting it though as it serves as proof. It's not that easy to define what falls under copyright, the most basic description would be "intellectual work". The exact scope gets important when somebody copies your work, but then you should contact an attorney anyways. Plus, google will give you lots of articles explaining it very detailed and more or less understandable.
You do not have to pay for your gui to be protected by copyright - to the extent copyright protection applies. Registering it probably has some minor fee.
To get copyright protection, publishing your GUI asap is essential as you need proof that you had it first. Consider sending yourself a dated - unopened - letter with a printed copy and as much information as possible. Keep it unopened until needed (and then open it in court or with professional help only - so make a copy for yourself, else you will forget what it says inside).
Protects novel designs for known stuff. Wikipedia says its a protection for the "ornamental design of a functional item" and includes computer symbols namely. This costs some money but offers more protection than pure copyright and can be applied to the whole GUI including some basic variations. Writing the application on your own is cheaper, but the result will be cheaper, too. (Consider a patent attorney something like a programmer, would you programm you security features yourself if you had no idea how to do that?)
If you want a design patent, keep the design secret until the application was submitted to the uspto.
This is issued for novel and inventive "inventions". Software patents are complicated. The short version is, you might be eligible to get this if your gui has some inventive (and novel) functionality or a novel and inventive combination of priorly known functionality with some new and unexpected result. (Inventive means it's not obvious for somebody skilled in the art, eg. a programmer with much experience - this is regularly fought over, so no exact definition concerning any specific case is possible).
This is the version that costs most money. I'd strongly advise consulting a patent attorney as utility patents, especially software patents, are difficult to write and prone to errors that you can't foresee without many years of experience. Plus, you might pay good money for a bad patent and notice only years later when you try to enforce it and it gets shred apart.
(Without knowing to much, but your question sounds like you are not getting this as you might be missing inventiveness.)
Also, if you want this, keep everything remotely interesting secret until the application is at the uspto.
With professional help, the application for a design or utility patent can be done in a few weeks (days even), but it can take longer too. That basically depends on your patent attorney, his schedule and might depend on your willingness to pay him to reserve you more time on his schedule.