Rabbids Big Bang

The jetpack has 10 levels. Each has 5 tiers. The cost of each tier in a level is the same (for example, the 5 tiers in Level 5 is about 180 coins). As such, there are 50 tiers for the jetpack. You need about 11345 coins. When you upgrade the jetpack to Tier 5 on Level 10, known as the max level, the achievement will be yours. To get coins relatively faster, you can redo levels in earlier (easier worlds). Try and reach the goal with 3 stars left on the time. Doing so will give you more coins.

This achievement is buggy. It may unlock even before you fully upgrade the backpack. That happened for me.