diff --git a/bin/billing_monthly_cronjob b/bin/billing_monthly_cronjob
index 8068218ebcfc4c3db83a0221102e39b95eb812a5..265fd9ad6bc247ad465d149c81eb5fa42ca223be 100755
--- a/bin/billing_monthly_cronjob
+++ b/bin/billing_monthly_cronjob
@@ -186,7 +186,7 @@ class ExpiredCustomer
 
 		def renewal_notification
 			"Failed to renew account for #{tel}, " \
-			"balance of #{@row['balance']} is too low. " \
+			"balance of $#{"%.4f" % @row['balance']} is too low. " \
 			"To keep your number, please buy more credit soon. " \
 			"#{btc_addresses_for_notification}"
 		end