100 000 tickets per case
Every case splits 100 000 tickets between its items. The odds you see on a case page are simply an item's tickets divided by 100 000. Cheap items own most of the tickets; knives and gloves own a handful.
Odds are set so that each item contributes a similar share of the case's expected value. That is why a skin worth a thousand times more than another is roughly a thousand times rarer.
What RTP means
Expected value (EV) is the average value of what comes out of a case if you opened it an enormous number of times. RTP — return to player — is EV divided by the price. Paid cases on DragonDrop are built to about 90 % RTP: on average, a case worth 10 coins returns items worth about 9.
The missing 10 % is the house edge. Averages only show up over thousands of opens; any single open can be far above or far below. The Transparency page lists the EV and RTP of every case.
The honest takeaway
Over time you are expected to lose about a tenth of what you spend. Case opening is entertainment, not an investment. On DragonDrop it's also a demo: coins are virtual and have no cash value.