How X API pricing works
X sells API access pay-per-use: you buy credits up front and every call that creates or reads something takes its price from them. There are no monthly tiers to pick from, so the bill depends only on what your app does. This calculator multiplies each count by X's unit price and adds the rows up.
What a link costs
The biggest surprise for most apps is the link price. A plain post costs $0.015, but a post with a URL costs $0.200. If an app posts 1,000 times a month and 20% of those posts carry a link, the 800 plain posts cost $12 and the 200 link posts cost $40, so $37 of the $52 bill is the link premium. How many posts carry a link changes the bill more than anything else.
Reads add up too
Reads are cheaper per item but come in larger numbers. Reading 100,000 posts costs $500. Owned reads, the resources of your own account, are priced lower at $0.001. X lists a cap of 3,000,000 post reads a month on pay-per-use, and the calculator tells you when your estimate goes above it.
Prices change
X has changed its API prices several times. The table shows the date the prices were last checked against X's pricing page; check that page before you budget, and tell us if a number is out of date.