Depending on where you live, how often you use the top (cycles up/down) and where the car resides (especially during the day), will all influence the duration of the top.
I am getting my top replaced next week. Prices ranged from $800 - $1150. Most of the reputable installers told me expect 4-5 years max based on the criteria above (for me). Mine is hitting 5 now. The installer that will be doing mine told me to get the best brand (in his and other installers opinion) from the net to save money and he will charge labor. End result = $700.
Last remarks, first vert I have owned. Love it. I think I actually worn the top out from use (the top gets raised/lower an average of 5 times a week all year). I drive home from work about 2-3 days a week with it down, on the weekends, and then 1-2 evenings. Just the other day it was down (in December) since the high was 75 and sunny.