fix fees for small length

runtime/common/src/fees.rs

@@ -21,8 +21,8 @@
 /// When the current block's weight and length are below the targeted thresholds, no fees are charged,
 /// as the weight-to-fee conversion results in zero. Once the block's weight and length exceed these
 /// targets, the weight-to-fee conversion maps 5 (5cG) to a base extrinsic weight.
-/// Additionally, a fee is applied based on the length of the extrinsic and is mapped linearly:
-/// 1_000 (10G) corresponds to an extrinsic length of 3.5 kilobytes and will be applied only if the extrinsic
+/// Additionally, a fee is applied based on the length of the extrinsic and is mapped affinely:
+/// 2_000 (20G) corresponds to an extrinsic length of 3.5 kilobytes and will be applied only if the extrinsic
 /// exceeds 256 bytes or if the block target in weight or length is surpassed.
 ///
 /// To further deter abuse, if the previous block's weight or length  the target thresholds,
@@ -103,7 +103,7 @@ where
         {
             0u32.into()
         } else {
-            Self::Balance::saturated_from(length_in_bytes.ref_time() / 350u64)
+            Self::Balance::saturated_from(length_in_bytes.ref_time() / 350u64 + 5u64)
         }
     }