You were not being unreasonable to pay up on that occasion (the dinner), but to be honest, that should the last time.
Friendships are often a series of unequal transactions: each person "over-" or "under-pays" in sequence, with the friends taking it in turn to be "in debt" to one another; and "payments" are often made in different currencies (e.g. I drive somewhere, a friend buys me coffee/lunch; someone babysits, I do the shopping for someone, etc.).
What is your transaction with this woman? If she takes and expects you to spend loads of money on her, how does she show her appreciation for you? Is she a good guest (thank yous, makes real effort to be amusing at dinner parties); does she babysit? Or does she just pick the most expensive restaurant and hand you the bill?
If you are feeling scared about telling her the bill business has to stop or be scaled down (but - no matter what - be agreed beforehand), just remember that if she strops off too badly, you won't have to face her again until she's apologised! (if she strops, she should be far too ashamed to face you again without an apology).
So you will be off the hook - one way or the other - as soon as you talk to her about this.
P.S. If she's sorrowful, "wanting to spend time with you, but not able to afford to go out to dinner unless you pay", there are plenty of home-packed, cheapie-cafe and home-hosted options. You can also counter with the fact that you felt hurt when she picked the expensive place and handed you the bill. Doesn't she like you for yourself?
. Don't let her reflect your own pain as hers!