Both rental and purchase prices are negotiable, even if the ad says “firm”.
Just like with an eBay auction, you should decide in advance what you are willing to pay.
How can you decide what price is the right price? Use the ideas in the "how to sell" or "how to rent" sticky threads at the top of the list of topics for this Buying, Selling, Renting forum to determine a price you are comfortable with.
Then make an offer, somewhat less than what you are willing to pay. Don't make it too low or a prospect that would eventually agree to something within your price range might not bother to respond. Otherwise there's no magic to what that first offer should be. By starting at a point a bit lower than your highest price, you give the seller/renter a chance to "win" the negotiation game by getting you to pay a higher price than your initial offer.
Making offers is a lot like going door-to-door trying to sell a product. Be prepared for your offer to be rejected, ignored, laughed at or ridiculed. Over and over again. If the seller/renter makes a counter offer, move up somewhat toward the highest price you are willing to pay. Eventually, if you are patient, you will likely find someone willing to meet your price. Unless....
If you have misjudged the market, you will figure out after a month or so of constant rejection that you might have to increase your offers for a better chance of success. How quickly you change your pricing strategy depends a lot on how willing you are to be too eager and spend more than is necessary!
Before you make an offer, be sure to ask enough questions so that you are reasonably sure what you would be getting (week #, maintenance fees, view, etc.) See the
TUG timeshare purchaser's checklist. But don't be a pain in the neck with too many questions or the seller might not bother to respond. For example, don't waste the seller/renter's time asking about amenities that you can find out about for yourself with a tiny amount of homework, simply by going to the resort's website. Or by Googling other sales/rentals at this resort.