adds cost of eroneous call (#2472)

Signed-off-by: m4sterbunny <harrie.bickle-ext@consensys.net>

Author: m4sterbunny

services/get-started/pricing/credit-cost.mdx

@@ -30,13 +30,15 @@ The process used to determine the credit cost for each request is as follows:
 
 ## Error code costs
 
-When making requests that return status code errors, some errors count towards your credit usage.
+When making requests that return HTTP status code errors, some errors count towards your credit usage:
 
 - **`429` errors**: These are not charged and can occur if you've exceeded your allowed throughput limit (requests per second).
 - **`402` errors**: These are not charged and can occur if you've exceeded your allowed daily credit usage.
 - **`4xx` errors**: These are errors caused by human input, and consume 5 credits.
 - **`5xx` errors**: These are server errors, and do not consume any credit charges.
 
+Making requests to a non-existent JSON-RPC method costs 160 credits.
+
 ## Ethereum
 
 The Ethereum methods serve as a baseline for pricing RPC methods. If a credit cost is not available for