I agree 150% with Peaches-and-Cream!
I'm planning a wedding right now, and I'd rather have one of my attendants tell me up front that they can't afford/don't want to/for some reason can't stand up, than to have to try to replace him or her down the road. The further into the planning process you get, the more difficult it is to try to replace a BM.
In addition to that, if you don't think that you'll be able to meet the time and financial requirements that being a BM will require, it's important that you tell her so. If you do decide to stand up and tell her that everything is hunky-dory, but then can't afford your dress, or your shoes, or whatever, you're going to cause her a lot of headaches.
Moral of the story? I don't think most brides would be insulted if you said no. Just because you don't/ can't stand up doesn't mean you love her any less... it may mean you love her
more!