I wouldn't necessarily assume that her motives are financial. If she's normally the type to duck out of bills then maybe, but otherwise it could easily be that she just doesn't like being on her own in the taxi. People do get ideas about things like that, whether they seem reasonable to others or not. It wouldn't bother me, but there are other things I'd rather do or not do that others might find odd.
If you're not comfortable with being alone in the taxi either, however, then you need to find a way round it - maybe take turns, or see if you can get a firm that sends female drivers.
If she is the 'last to the bar and first out of the door' type, then you could ask for a fixed price taxi and get her to pay half when she gets in (or before you leave).
Also, does she have a husband who could take his turn in dropping you off? It doesn't seem fair that you organise the outward journey every time.