Here's the problem.
Realistically, I am guessing that Susan is unlikely to organise a taxi twice a week, as it would be expensive for anyone but certainly for someone who already has a tight budget. It also doesn't look like she has many other options to get there and back. You are entirely within your rights to refuse her the lift (and I understand your reasons for wanting to do this), but that action will have consequences.
Consequence 1: Not having a lift that may mean Susan is no longer able to attend the gatherings on the weekends or, at least, no longer able to attend so frequently. She may also be upset and/or angry. You will definitely lose her company, and you may also lose her as a friend.
Making friends is hard as an adult, particularly if you don't have much money for socialising and live in a small area. Are you ok with losing her as a friend entirely?
Consequence 2: Once Jane finds out that you've 'dumped' giving Susan lifts, she will also have her own reaction. From the replies you've received on the post so far, you can probably already tell that some people think you are right to refuse to drive Susan home but others think it is unreasonable. If Jane sides with Susan and is angry with you, you may also lose Jane as a friend too.
Would you also be ok with that? You would then be left with no friends at all, and nothing to do on a weekend.
To reiterate, you absolutely could refuse to give her the lift. You would save yourself some petrol money and some time. But would it really be worth it?