First, I'm a little confused on who is actually HOSTING this shower. Is the maid of honor hosting the shower or all of you bridesmaids hosting the shower together? If the answer is the MOH is hosting on her own, then she's in bad taste to even ask you guys to chip in to pay - big shot job salary or not, but I digress.
I'm going to assume this luncheon is at a restaurant since you're wondering if guests can pay. If formal invites are being sent out, the host(s) is(are) generally responsible for the cost of a party they're throwing (with the exception of excessive booze), not the guests. Keep in mind, many places won't do separate checks for large parties, so depending on where you are and how many people you have, having everyone pay their way may not even be an option, plus never mind the social nightmare of having a bunch of women trying to figure out how much they owe off of one big bill at the end of the party. *lol
If this luncheon is happening in someone's home or private location, and is being catered, then guest should not pay a dime under any circumstance.
If all of you girls are hosting together, I would call (not email) the MOH and explain to her that right now money is tight and you're not budgeted to spend $100 on the shower (especially if you've got other expenses to plan for with this wedding like alterations or hair/make-up, etc). It doesn't really sound like any sort of Plan of Action was talked about or budget was set, so you've got a bit of a leg to stand on here, IMO. Offer your services in another way that you can - doing the invites, following up RSVPs, set up, tracking the "gifts from whom" for the bride, whatever, just offer help. You don't need to go into the details of your personal financial situation, it's not her business. :) Something can be scaled back in cost to compensate your share or other girls can pony up the cash, especially if they are making bank & you're doing extra helping.
Hope this helps!