Legal Question in Business Law in California

Yes, she can sue you. Suing someone else is very easy, and if done in Small Claims Court, where the dollar limit is generally $7,500, it's also quick and cheap. If you are sued, you'll have to defend, or face the likelihood of losing by default.

However, you didn't ask the rreal questions here, which are (1) will she sue? and (2) will she win? The answers to these questions are not "yes;" "maybe" is probably the best answer, with "maybe not" being a good alternative answer, especially to #2.

Here's my legal analysis: The model performed her services, and is entitled to some compensation. If no agreement was reached, but she showed up for the shooting session with full knowledge of what you customarily pay, a judge or jury is likely to believe she impliedly consented to your last and best offer.

If you are concerned about fairness to her, or about being sued, send her a check for the amount of you best pre-shoot offer, marking it conspicuously as "Tendered as payment in full for photo shoot 7/15/2011" or words to that effect. This is not foolproof, as she can cross it out (see Civil Code section 1526(a)), but I'd recommend giving it a try.