Rate per 1000
Rate per 1000 is the price a panel lists for a service, stated as the cost of 1,000 units of the action, whether followers, likes, views or comments.
The charge for an order is the rate times the quantity divided by 1,000. Take an illustrative rate of 4 balance units per 1,000: an order of 500 costs 4 times 500 divided by 1,000, which is 2 units; an order of 2,500 costs 10 units; an order of 50 costs 0.2 units. The order form shows the calculated charge, and it is deducted from your balance when the order is accepted.
The arithmetic has edges. Panels store charges to a fixed number of decimal places, so very small orders on low rates can round, and the service minimum sets a floor on what an order costs. The unit also varies by service: for comments it is one comment, for watch hour services it is usually one hour, so the rate is per 1,000 hours. Read what the 1,000 counts before comparing two services.
Comparing rates only works when the services are comparable, and the rate alone does not tell you that. A lower rate on a service with no refill can cost more in the end than a higher rate with a working refill, once you pay again to replace drops. Rates also change without notice when providers change theirs. An order already placed normally keeps the charge it was accepted at, while new orders use the new rate. If you order through the API, the services action returns the current rate, so re-read it before large orders instead of trusting a stored figure.
What it does not mean
Rate per 1000 is not the price of an order, and it does not mean you must order 1,000. You can order any quantity between the service's minimum and maximum and pay the proportional share.