It's not about the prize's its about the challenge to go out side the normal competitive spam lists that pop up everywhere.
Even if I were to win a prize it would not equal the amount of money spend to get that prize. For me it's about playing people in different area's and seeing if I can win with a comp army, I know I could crush the majority of my opponents with my IG but where's the fun in that?
Not to mention some of the top 40k players in the country are going to be there, and they are ignoring the comp system. It's always good to take your army outside of the local pool and swim with the big fish. If I can trash them with my comp tau list thats a win for me.