I'm now thinking of getting 600 NIS. I figure this will cover the sherut from TLV to Jerusalem and other cash expenses over a three-week stay, with more cash easily available if necessary.
I have four days in Jerusalem, followed by two weeks of business in Petah Tiqva, then another week on my own before my return. A visit to Jericho and a friend in Mitzpe Yeriho and a trip up to Rosh Hanikra look appealing. On the off-chance that the weather isn't too brutal I might take a trip to the Negev.