My experience has been that you should receive the promised "bribe" for attending a timeshare presentation. You don't have to make an actual purchase to receive the gift. But I'm surprised at the amount you say that you've been offered. A more typical gift might be on the order of either a $75-100 gift card or 10,000 to 15,000 Marriott points.
I'd listen to what they say and, if you are not interested, a polite but firm no should suffice. Some timeshare salespeople are pushier than others, of course, so a few nos is sometimes needed. Expect that they might tell you that you've been given a special, limited time opportunity, and the offer is only good on the day of the presentation or for a very limited time afterwards. They'll say that if you don't act now, prices will be going up.