Can't say EXACT numbers, but by doing my front grill block (1 chunk at a time), each time I blocked more, I noticed I was able to coast a little further both city and highway. Even though city is a lot of lower MPH, it was noticeable.
According to the wiki...
Car MPG Efficiency Modifications Main - EcoModder
2.89% average gain in MPG was observed with partial or full frontal block. This, of course, varies from car to car and environment to environment.
3.56% average gain in PMG was observed with smooth wheel covers. This, of course, varies through different wheels, tires, offset of wheels, car, and environment.
Another tip, if you don't mind a bit of "ghetto", yet easy and cheap, is if you can find duct tape that matches your car's paint, you can use that tape to cover up anything on the front fascia for your grill blocks. Larger openings, use a backer like corrugated plastic. I had to use the backer quite a bit to fill in fog light holes and such on my neon. This allows you to constantly add to it and find the "sweet spot" for your cooling needs. Then, later, if you got ambitious... can do your fiberglassing and painting.