Factually, there's no money we can afford to lose as long as we work for it. We wouldn't have bothered to earn it if we can afford to lose it. That statement "money you can afford to lose" is just a figurative statement that virtually refer to money that will not have much impact on you if you get rid of it. People plan ahead before even receiving their salary and you could see it is almost impossible for one to have money they can afford to lose. However, there are money that have less important things attached to it and losing part of it will not be felt with severe emotional cripple.
No matter how little the amount is, you might feel bad thou but not compare to when you use money meant for most important things like feeding, rent and others. Again, use money you can afford to lose.