After completing the quest Reilly's Rangers and offering Reilly your help with their mapping service, she will give you a Geomapper module. As long as you have the module in your inventory Reilly will pay you 30 caps per location every time you return to the Ranger Compound and talk to her. (You do NOT need to carry the module everywhere; you just need it when talking to Reilly.) Don't forget to speak to her right away, after receiving the module. She will pay you for the locations which you already have on your map, even those that you have already discovered beforehand, making this quest lucrative very quickly.
Also, if taken early on, it can be extremely profitable.
It is also possible to exploit the Explorer perk to quickly capitalize on this quest, despite the fact that locations revealed with the Explorer perk are not technically discovered.
pcps3xbox360 It is possible that you won't get paid for locations you already have the first time you talk to Reilly after she gives you the Geomapper module. She will say that the Rangers already know about all of the locations you have and will give you nothing. If this happens, try immediately talking to her again. You can also try dropping the Geomapper module and picking it back up, then speaking with Reilly again. If she still won't pay you for the locations you have already discovered, try leaving the Ranger Compound and re-entering before speaking to her again. Another alternative is to simply seek out at least one new point of interest and then return. [verified]
ps3xbox360 Sometimes, if you talk to Reilly and ask for the caps for the geomapping, she will say that she will give you the caps, but will make her way to her bed to sleep. When you wake her up, she will say that she already has all those locations and will not pay you. This can be fixed by reloading a previous save. [verified]
ps3 Sometimes Reilly will claim the Rangers already know about all of the locations you have and refuse to pay, even if you exhaust all of the options listed in the first bug. [verified]