Mastery of level one equals 167 harvested squares, or about 7% of the total harvests needed to master level 4. When you master level 1, you receive $2000 coins - worth additional 11.98 coins per each previously harvested square. (You also receive 20 XP.)

Level 2 requires 500 harvests - 20% of level 4 mastery. No additional coins, but a gem and 50 XP.

Level 3 requires 1,167 harvests - 47% of level 4 mastery. You receive $4000 coins - worth additional
$1.33 coins per harvested square from level 2 to level 3. (Also, 100 extra XP) This averages out to about $1.714 extra coins per harvested square from the beginning until mastery of level 3.

Level 4 requires 2,500 harvests - 100%. No additional coins, but a second gem, 200 XP, and a female gnome.

After you master level 4, every harvest gives you an extra $5 coins per harvested square.