Agricola question about minor improvement and occupation cards
You pay the cost a single time when you play the card. That is, you take an action that allows you to play an occupation or minor improvement, pay the cost, take the card from your (secret) hand and put it face up on the table.

Note that in your question, you seem to have conflated "play the card" and "use the card". In the rules, "play" is used as I have used it: playing the card from your hand. If you later use the card (i.e. take advantage of it) you're not playing it. You might have less confusion if you think of subsequent use of the card as using it, not playing it.

In any case, once you've paid and played the card, you're done paying. The card likely has some kind of ongoing benefit, and you don't have to pay again to get that benefit. For example, once you've played Field Watchman, which lets you plow a field when you take a grain, you get that field plowed for free whenever you take a grain.

To make this feel really obvious, just look at the major improvements. You definitely don't have to spend clay every time you want to use your fireplace!