Google Maps API, does adding a marker cost anything towards usage limit?

This is bit to long for a comment

The number of free maps load is calculated for each time you reload a page .. alias when a new request for google maps apis is send to google ..

then if you add a marker using a ajax and create marker without a new load of HTML page you dont cost ... if you perform a reload for add the marker .. you cost.

There is an accurate explanation regarding map loads in the FAQ section of Maps APIs:

A single map load is charged when any of the following occur:

• A web page or application displays a map using the Google Maps JavaScript API.
• A web page or application displays a Google Street View Image API panorama using the Google Maps JavaScript API. If a Street View panorama replaces a map in the same div element, the panorama is not charged.
• An application requests a single map image from the Google Static Maps API.
• An application requests a single panorama image from the Google Street View Image API.

https://developers.google.com/maps/faq#usage_mapload

As you can see adding a marker is not charged. Only new google.maps.Map(element, options) or new google.maps.StreetViewPanorama(element, options) can incur the map load in Maps JavaScript API.

Referring to storing data, terms of service allows to cache data for 30 days. Have a look at section 10.5 (d). It says:

No caching or storage. You will not pre-fetch, cache, index, or store any Content to be used outside the Service, except that you may store limited amounts of Content solely for the purpose of improving the performance of your Maps API Implementation due to network latency (and not for the purpose of preventing Google from accurately tracking usage), and only if such storage:

• is temporary (and in no event more than 30 calendar days);
• is secure;
• does not manipulate or aggregate any part of the Content or Service; and
• does not modify attribution in any way.

https://developers.google.com/maps/terms?#section_10_5

Also, note that place IDs are exempt from the caching restrictions, so you can store them without time limit:

https://developers.google.com/places/place-id#save-id

I hope my answer clarifies your doubt!