That only explains the particular query plan chosen for the query at that time. This might vary depending on the table statistics, query parameter or moon phase if you're unlucky. It is a very important tool, but doesn't solve the problem here that in the end the database is in control and decides how many rows to read, not you.
This is a good point. You can control for some variables (like query parameters), via testing, however as table statistics change, so does query planning. For instance, your table gets bigger (or smaller) and MySQL chooses a different query plan.
There's almost no way to completely predict pricing in all but the simplest of apps. Overall, the pricing model seems untenable.
