To be honest I think the GUI will have as much effect on the price of monero as tiddlywinks has on the price of wheat.
I think it is important to distinguish between short term and longer term effects of an out of the box GUI.
Short term there might be some speculation that the price should rise, but that's it. It likely won't make a difference to the current Monero users, since they are already comfortable with the CLI.
However longer term, a hurdle for new users is removed that do not have the patience / knowledge / interest etc to learn a 90's DOS like interface. They want a graphical interface with just a few options. No geeky stuff to distract them from the core functionality (look at all 'modern' mainstream web pages).
To the power user the CLI will likely stay the preferred way. Let the GUI be for convenience and ease of use to appeal to the not so tech savvy users. This will attract new users, hence increasing demand.