like anything - supply-demand. you need to bring new people into the market or make the existing buyers want more.
I can definitely see the inevitable movement of GPU number crunching power shifting from Litecoin to Primecoin market as soon the a GPU miner becomes available publically. It's just a natural order of economical distribution. Litecoin rose int he midst of Bitcoin's GPU mining fall, and so will Primecoin do to Litecoin. The only thing we've yet to witness is an Asic Scrypt miner to make that transition complete.
And since there will be many more people involved when GPU mining is possible, the demand will naturally rise due to more people involved with the coin. Just like Bitcoin, its price is only as high the the people holding are willing to sell at all.
Prices can be crushed down by a few hoarders and dumpers, but to rise consistently, it needs
adoption rate in quantity.
Pretty sure that the GPU miner for Primecoin won't be any faster. It's finding prime numbers, using basic arithmetic. This is what CPUs excel at.