We bought a TT membership on a resale deal a couple years ago. Paid much less than they now ask and there is no per night fee. It also has some perks that are not now available. Our annual dues are $550 for 50 nights per year, so it works out to $11/night if we use them all. I've seen memberships for sale for as little as $100. There are lots of different types of TT memberships with different rules and perks, so look around IF you think that sort of thing would suit you.
You generally don't have to pay the annual dues if you don't want to, but you lose your initial investment if you just stop paying. You can't sell it to anybody else if the dues are not current.
The main argument for NOT buying into TT(or similar) is the parks may not be where you want to go. If they are, and you use will them regularly, it's a no brainer. If you are not sure, don't buy because it is a long term commitment.
Former owner of 2004 American Tradition and several other RVs
Home is West Palm Beach, FL