Each time You level up, You receive certain amount of bonus coins: 15 on lower levels and 20 on higher levels.
When you buy very valuable property, you can gain more than one Level at once, i.e. going from level 14 to 16. In that case You will receive 30 coins. BUT the maximum reward for "skipping" levels can be equal 3 times level up reward. So if You go from level 16 to 20, You will still receive only 45 coins.