What was the average number of buses passing through the toll plaza per week?

First, let's add all bus numbers (in hundreds):

$28.3 + 24.5 + 20.4 + 24.4 = 97.6$ (hundreds)

Average number of buses = $\frac{97.6}{4}$ = $24.4$ (hundreds)

Converting to actual number: $24.4 × 100 = 2440$ buses

Therefore, the average number of buses passing through the toll plaza per week was 2440.