I agree with the other posts. Simply tell them that you would LOVE to (for those that you really want to be involved) and are honored, but unfortunately given current circumstances cannot afford to participate in this capacity. One thing that can be helpful is to offer to help decorate for the reception, assemble little table gifts, etc. I have done this in the capacity of a bridesmaid, though not all bridesmaids did so. This was always very helpful as things do not always go smoothly and sometimes having another person with ideas, vehicle, and time to run an errand is a godsend. Since time is likely going to be an issue as a limited commodity as is money, choose to put your efforts towards those weddings where you feel the closest bonds-->prioritize.
Finally, keep in mind that while it may be a "burden" to have so many weddings, in my family it has always been said that weddings are a much better way to visit with family than funerals. So, with that perspective enjoy these happy occasions.