It all depends on the needs of the project.
Do you just need to know where each vehicle is?
Will you need to know the route traveled?
Are all devices Android? Or do you have IOS?
How many vehicles will use it?
How often do you need the vehicle update?
Is there a battery problem in the devices?
In my case, I would budget between 2500 euros up to 10,000... or more
Then this system will have a monthly cost, since the data must be collected on a server.
Do you have your own server to avoid this cost?
It is difficult to say a price without having well-defined specifications.
Technically FIREBASE messages can be used to request the position every 15 minutes or something like that.