Black Lion Ticket Weapons
They are usually available at the price of 1 ticket for one month, then they get replaced by either a new set or an older one depending on the events (usually a new set). When they get replaced, they are still available but at a price of 5 tickets. But you can buy them all year round from the TP with gold, which is much more reliable than buying keys to open chests hoping for a ticket. Skins usually cost ~60-90g when they come out, then go slowly up, so if you want to buy with RL cash, I think buying 800 gems and converting them to gold should suffice for one skin (not really sure on the exchange rate atm).
Note however that 4 sets are about to (may have just?) go away for who knows how long, in order to make room.
You should generally assume that any large Tuesday patch may bump up the ticket price, and collect asap any at the one ticket price that you will really be miffed to miss out on.