Well yes, you can get your own Google Maps API key and for a single user with moderate use it would probably not hit the threshold to be charged by Google Cloud. However, this just gives you the ability to use the Javascript API (or others) to retrieve a map. Effectively it’s no different than using the free google maps in your browser. In order for it to be useful with MSFS, you would need to write your own application to read and interpret MSFS flight plans. Then you need to translate the MSFS waypoint coordinate data from hours, minutes and seconds to 12-digit GPS to place them as push pins on the map and finally draw the vectors for the route. If you want to calculate distances between waypoints on a sphere (Earth) using GPS coodinates, that will require several other conversions and some additional math.
In fact, an MSFS flight plan actually contains multiple flight plans in all cases (aircraft and active at least), but may also have ancillary flight plans contained in the same file (ATC/IFR, Bush Trip, etc.). Flightplan maintains all of the flight plans contained in the file, modifies the appropriate ones when waypoints are changed and updates any values necessary for MSFS to recognize your flight plan when it loads. Total number of waypoints, aircraft position and arrival/departure ICAO are just a few examples.
Additionally, Flightplan includes the tools to manipulate the route (add, move, modify or delete pushpins) on the map itself, save those changes in the appropriate format for MSFS to recognize and generate the .FLT and .PLN files to be loaded in World Map or from the cockpit. When in flight, Cockpit Map communicates with MSFS real-time to get aircraft data and translate those to a moving aircraft icon on the map pointed in the right direction. In summary, it takes a substantial amount of code written in several programming languages to utilize the Google API key to do something useful. It is actually intended for use by web developers to create interactive maps on websites, Flightplan is utilizing the API in ways I’m sure Google never conceived of.
The core Flightplan application is a stand-alone Windows Flight Planner (MSFS does not need to be running) with a variety of tools for modifying flight plans generated in Microsoft Flight Simulator to control spawn location, aircraft state and position, weather, time of day, fuel levels, selected runways and much more. Technically the map features are optional, all of the above can be manipulated without using a map. Usage of Flightplan itself and all upgrades and updates are free for life, but obviously one of the big benefits of Flightplan is the drag-and-drop google map interface and the in-flight Cockpit Map, which requires the usage of the Google Maps API.
Flightplan uses the Google API key issued to simflightplan, and we are responsible for the cost of all usage. This is why a subscription is required after the 14 day trial to continue using the map features. Simflightplan is charged for each map rendered, so the subscriptions cover the cost of varying usage each month.