Farming can be an extremely expensive skill at the mid to late stages. This is because ready to plant saplings and ultra compost cost a lot of GP. Players who want to achieve level 99 Farming as soon as possible can expect to pay 60-100 million GP. The amount varies on Grand Exchange prices. Alternatively, you can plant fruit trees instead till level 99 Farming. This will minimize the amount you spend on the skill. However, it will take considerably longer to achieve level 99 mastery.