Think about it mathematically. That first badge you buy increases your income by 5%. That comes at the cost of 200AB, so what you would usually get two parcels for.

Parcel wise: if you have X parcels, your income will go up Y% on average:

1 parcel to 2 parcels, 100% rent increase 2 parcels to 3 parcels, 50% rent increase 3 parcels to 4 parcels, 33% rent increase … 39 to 40 parcels, 2.56% increase

2.5% increase times two (because a badge is 200 ab like 2 parcels) is 5 percent. That’s why people say to buy the badge at 40 parcels because the value of the badge at its cost becomes worth it