Due to the new achievements added in the last few updates, you can now afford all five builders. Completing all the achievements will now give you 6,432 gems, enough to buy all 5 builders. However, some of the high-yielding achievements such as "Unbreakable" and "Firefighter", are very tough to achieve for most people. So I am going to calculate how long it would take you to get all 5 builders on removing obstacles alone.
Every 8 hours, a new obstacle spawns in your village (Provided that you do not have 40 obstacles already). When you remove an obstacle, you have an equal chance of receiving 0, 1, 2, or 3 gems per obstacle.
Expected Value = 0 (0.25) + 1 (0.25) + 2 (0.25) + 3 (0.25)
Expected Value = 0 + 0.25 + 0.5 + 0.75
Expected Value = 1.5 gems
On average, over the course of a long time, you would get an average of 1.5 gems per tree, bush, or trunk removed. Let's see how long it would take you to get a third, fourth, or fifth builder on removing obstacles alone.
Third Builder: 500 gems
500 gems / 1.5 gems = 333.33 obstacles needed to be removed (On average).
333.33 obstacles * 8 hours = 2,666.67 hours
2,666.67 hours / 24 hours = 111.11 days
111.11 days = 3.7 months (approximately)
Fourth Builder: 1000 gems
1000 gems / 1.5 gems = 666.67 obstacles needed to be removed (On average).
666.67 obstacles * 8 hours = 5,333.33 hours
5,333.33 hours / 24 hours = 222.22 days
222.22 days = 7.4 months (approximately)
Fifth Builder: 2000 gems
2000 gems / 1.5 gems = 1,333.33 obstacles needed to be removed (On average).
1,333.33 obstacles * 8 hours = 10,666.67 hours
10,666.67 hours / 24 hours = 444.44 days
444.44 days = 14.8 months
To get all three extra builders, you will need to play for 777.78 days, which is around 25.9 months.
Achievements are good, aren't they?
Here's a little extra thing:
How about the XP points you will get for removing all these obstacles? The possible obstacles that will spawn are: Mushroom, Bush, Trunk (1), Trunk (2), Trunk (Fallen), Tree (Small), Tree (Medium), Tree (Large).
Expected Value = 3 (0.1) + 3 (0.2) + 5 (0.1) + 5 (0.1) + 5 (0.2) + 5 (0.2) + 6 (0.05) + 7 (0.05)
Expected Value = 0.3 + 0.6 + 0.5 + 0.5 + 1.0 + 1.0 + 0.3 + 0.35
Expected Value = 4.55 XP
4.55 * (333.33 + 666.67 + 1,333.33) = 10,616.6515 XP points.
That's a lot of XP points. :)

