I pay if I like the game after I play a free trial or if the game has amazing graphics and gameplay. I usually will buy any game that pushes the hardware that I am currently using. I don't like Grand Theft Auto, even for consoles but I bought it just to see how it runs on my 1020 and future devices. I don't mind paying to play and I try to keep around $15 set aside for games over a 3-6 month span of time.
Some games, ones that I have been waiting on or that I am a huge fan of the genre are basically insta-buys, such as Asphalt 8, the combination of graphics, sound, controls and an established franchise that I actually like meant that I would have bought it even if it was $6.99 as I was eagerly awaiting it.
Build hype for your games and people will be more willing to buy them and yes, the free trial is a must and separates Windows Phone from other OSes.