@Randika De Silva I believe the charge for 25K transactions is 7$ and is applicable for AutoSuggest and SpellCheck API's for this tier. Please check the asterisk(*) and note at the bottom of page explaining the same.
For the Web Search API and other offerings in this tier the charges are 7$ for every 1000 transactions. This should mean if all calls are made to web search API then atleast $84 should have been incurred for exactly 12k transactions.
If you have any queries about billing you can always raise a support case from Azure portal to enquire about the billing even if you do not have a support plan for your subscription.
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