I think Roderigo is supposed to look the fool here. He has been soundly duped, and not only that, he's been beaten for his pains as well.
He's the type of character who allows the audience to feel good about ourselves...we laugh because he's *such* a fool, but we feel great because we know we're foolhardy, but we'd never be that dumb. Or so we wish to think...ha...ha.
Most of us would like to think that we'd see Iago coming and wise up after the first purse-full disappeared. The reality is that we're (most of us) more like Roderigo than we want to acknowledge. Whatever our unrealistic desires are, many of us have the potential to throw money away or to be tricked into believing that we can, with a little more time and money, achieve our ends.
So when we laugh, it's not entirely derisive. It's that laugh we laugh when we're exposed and we feel stupid. Yes, he's a fool and we feel sad for him, but we also feel a twinge of discomfort because there, but for the grace of the gods, we could go.