As per the session pricing documentation you will be billed for each autocomplete request seperately until 12 requests. After which it should go in the session usage plan.

• First 12 Autocomplete (New) requests: You are billed for each Autocomplete (New) request, up to a maximum of 12 requests, using the SKU: Autocomplete Requests.

• For Autocomplete (New) requests 13 and higher in the same session: You are billed at the SKU: Autocomplete Session Usage, meaning there is no charge for those requests.

Documentation: https://developers.google.com/maps/documentation/places/web-service/session-pricing

As per my understanding the session token handling is primarily meant to simplify billing into discrete groups. It might not reduce costs until you go searching more than 12 characters when searching with autocomplete.

Also after your autocomplete requests the session closes only when you make a Places Detail API call with the same session token. In which case the session token is marked as expired.

Now if you try to use the same session token without closing it for a different autocomplete request it will be considered as a separate request.

The documentation is not clear how google determines this but I think if any parameters other than input change it will be considered as a new session.

Be sure to pass a unique session token for each new session. Using the same token for more than one session will result in each request being billed individually.

Documentation: https://developers.google.com/maps/documentation/places/web-service/session-tokens