高い, 低い, and 安い: height, level, and price
Compare how 高い and 低い describe height, numerical values, or levels, and how 高い and 安い evaluate price. Since 低い can also describe a low price in some contexts, do not memorize only one opposite for 高い.
高い: high, tall, or expensive
高い describes physical height and a high value or level. 丘の上に高い塔が立っています。 refers to the height of a tower, while この町は夏の気温が高いです。 refers to a high temperature reading.
It can also mean expensive when applied to goods or services. この店の果物は少し高いです。 says that the fruit at this shop is somewhat expensive. The noun and situation show what kind of scale is high.
低い: low in position, value, or level
低い applies when a value is low on a scale such as height, position, pitch, temperature, or probability. この椅子は机より低いです。 and 今日は気温が低いです。 apply the same adjective to different scales.
It is also possible to say 価格が低い or 料金を低く設定する when discussing price as a numerical value. Do not claim that 低い is impossible in price contexts. For an everyday evaluation that an item costs little, however, 安い is more direct.
安い: low in cost
安い evaluates a good or service as inexpensive and low in cost. 駅前の店では野菜が安いです。 and この靴は思ったより安く買えました。 are representative examples.
The ordinary opposite of 背が高い is 背が低い, while the ordinary opposite of 値段が高い is 値段が安い. In statistical or policy language, 価格が低い can also be natural. Choose the wording that matches the scale and register.
Quick check: Which of 高い, 低い, and 安い directly expresses the evaluation in ‘I was able to buy these shoes for less than I expected’?
Show the answer and why
Use 安い: この靴は思ったより安く買えました。 The sentence evaluates the low cost of the shoes, not their physical height or position.