Flag value in col d based on value in col b

Sub checkb()

Dim ws As Worksheet 'to set worksheet
Dim lr As Long 'to find last used row in column B (assuming you want to check only cells with data
Dim i As Long

For Each ws In ActiveWorkbook.Worksheets
    ws.Activate
    MsgBox ws.Name
    lr = ws.Range("B" & Rows.Count).End(xlUp).Row
    For i = 1 To lr 'change 1 to the first row of data you wish to check
        If ws.Range("B" & i).Value < 30 Then ws.Range("D" & i).NumberFormat = "$#,##0.00"
    Next i
Next ws

End Sub

Works on my end, but it only works through rows where b is not blank.
Let me know if and when I can help more