Dilemma: You live in a different state than the bride and other bridesmaids, so you won't be able to attend a pre-wedding event. Surprise: The group still expects you to pay up for your share, anyway.
Brides don't want bridesmaid drama, and they are likely to be understanding if a far-away 'maid can't fly in for the shower. More often than not, it's the maid of honor and the other bridesmaids who will get angry, since they have to pay more for their share of the party now. Unfortunately, you might just have to deal with any tension and pettiness that arises (e.g. they omit your name from the invitation as a host).
What to say: "Although I can't attend the party and am so sad to miss it, I'd like to contribute (insert $X amount) to the party fund, to help you all put together a terrific celebration for the bride (one she’s going to love).”
You could also offer to help from a distance by researching invitation styles, catering prices, or whatever else the maid of honor needs assistance with. Many bridesmaids who can’t attend the shower will join in with the other bridesmaids to give a group gift from the registry. It's also a sweet idea to Skype the bride after the party and ask her about how it went.
But no, you don't have to pay a full share of the shower expenses if you can't afford it and won't be able to attend. It’ll raise some eyebrows, but you have to be true to yourself.