i know, but fixed amounts are the only way to give you some money for all other opportunities but Looting... Looting alone can be defined as a portion of GDP
btw, with next version, we will give all siege defenders +50% of ammunition. Defenders usually did prepare themselves for siege, having additional ammo distributed over walls and stations was common practice. It also means that ranged unit value will increase in sieges... even those simple stone slingers who tend to do minimal damage will now be able to shoot for much longer to deal some additional damage..