From ErfWiki

Proposed Canon

A Fund-razer is the conquering and sacking of a city for financial gain.^{First Intermission 49}

Speculation

The money gained is a function of the cost of building/upgrading the city in the first place.

Real World

Fund-razer is an obvious pun for "Fund-raiser", the activity of any group to go outside of itself to raise more funds for its activities.