When she said they wanted to extend the flower bed you should right at that point mentioned that the cost is going to be much higher or at least have thrown out a number (Like WEEZE said) or a range so that she would have an idea of how much it was going to cost. Saying something like "Extending the flower bed is going to double my original estimate" OR telling her that with just renting a tiller that is going to add $200 to the overall cost and the additional labor,etc could run another $500 or so. .
You do not tell them $250 and then slap a $1,000 as a final bill. Not good business. IT is HARD to build a good rep and VERY EASY to get a bad one in a town. Just a thought The loss of a few hundred now is better then getting SLAMMED in the future on future possible accounts. You will get known as the Ba estimate company.