First images of public properterty belong to the photographer. The gov't cannot prevent or stop the use of images of public(ly) owned property.
I have made several inquires along this line. Just like images that belong to
the National archives the military and several government agencies can;t charge for usage, just printing costs. Your tax dollars paid for the buildings, parks etc. Permits are sometimes needed for shooting in those areas, but the images belong to the photographer.
2nd the buyer of the images may understand some of usage rights. So the seller should act accordingly. Usage rights are sold so, that the end user can utilize the image legally.
Try not to sell all rights to the image, but limited and exclusive rights. If you set up the sale according to the usage + exclusive time frame.
You want to find out what type of material he wants to produce. Newspaper, magazine or TV is one set of rates, web based is additional.
The client will probably want a comprehensive usage agreement. If so something in 1200 to 1500 per year is not unusual. 250 to 400 would be for just one type of usage. As far as lowering the price against possible future work is basically BS. Enforce your rights, conduct your self in a professional manner and you will get respect. THAT will get you future business and allow you to negotiaite from a position of strength.
There are great calculators on the EP website along with some excellent contracts. EP has legal assistance from top attorneys so you can depend on the quality of their stuff.