40 turns may be ridiculous to repay a farm, but a) it repays itself, and b) it pays an amount equal to it's original cost every four years. If you're swimming in cash, it's not a problem, but if you had 10 cities with that farm producing 60 dinarii per turn, that's an extra 600/turn, which translates into another military unit per turn.

Another way to look at it is if you get that farm 100 turns into the game, over the next 400 turns, you will earn 24,000 dinarii more than you otherwise would.

It's not a huge amount, but it's something. Kinda like saving your change rather than throwing it away every time you buy groceries. Like starting a 401k plan, the earlier you get a farm going, the more money you will make over the course of the game.