I think that one big difference between being in kollel vs. working is that when someone is in kollel, I would imagine that it is much easier to live simply than when you are working.
When someone chooses to learn in Kollel, they are aware that they chose to sacrifice (in the physical realm) in order to live a more Ruchnius lifestyle. So they are ready and willing to live simply. That is a basic part of Kollel life. When someone chooses to work all day to make money, they expect to be able to not have to scrimp TOO much.
I know that personally, whenever I have to make a decision about taking on an extra job or extra work hours, when I make my cheshbonos, I always take into account the fact that the more I work, the more money I am likely to spend.