Yes, cost definitely plays a role in our decision. We'd certainly want the photographer to stay long enough to get all of the standard shots at the reception, (i.e. the first dance, mother/groom dance, father/bride dance, cake cutting, tossing bouquet, etc.) and go around to each table, get some shots of people dancing. i just don't want him to end up staying there sooo long that the party dies down, most of the people have left, dinner is long gone...yet he still has a few hours left because hey, we paid for it!
Basically, I wanted to get an idea of how long the typical day would last: from pre-ceremony...ceremony...cocktail hour (where we'd be taking the group photos)...to a few hours into the reception. if all of that easily adds up to 7 hours, then i guess we'd consider a 7 hour photo package. But if all of those things typically take, say, 5 hours, then we'd opt for a different package.
I figure a lot of our family and other guests will have cameras to take endless candid shots that we can all share with each other.