Money. Wargaming is a great hobby in that there's something for all pockets. As with all things quality does cost but there really is something for all. I'm in a fortunate situation where I have a reasonable amount of disposable income.
Space. A hobby that involves collecting (as wargaming does) will require space to store the collection. Wargaming is very flexible in this regard as there are so many scales to choose from and also you can choose to game at skirmish right through to grand battle. Because of my eclectic and diverse interests I'm reaching the point where I have to do something about the storage situation.
Time. The key factor for any activity. Additionally, how much time you spend on an activity dictates how proficient you become and the wargaming hobby is no exception to this. The experience and proficiency you gain often then saves you time. I spend 11 hours or so out of the house each day so my hobby time is restricted.
As I look to add to my collection I look at items that are of a decent quality as I can't be wasting my most precious resource (time) on something that doesn't grab me. Gaming forms a small proportion of my hobby, so I have to really enjoy the kit I'm building or the figure I'm painting. Just being able to put something on the table isn't what it's all about for me. I prefer figures to have more realistic proportions which means that many ranges just don't appeal to me.
Having said all the above I still try to get a value for money out of my purchases. For quite some time now I probably spend as much researching as I do hobbying to try to ensure that what I buy is right for me and enhances my collection. And then I need to find a place to store the new item...