What would Halloween be without eerily-lit, grinning jack-o'-lanterns in the windows to greet trick-or-treaters? Plenty of families throughout the Midwest are going to find out, because the summer's brutal heat and humidity have devastated this year's pumpkin crop, driving up prices. Wholesale prices for jack-o'-lantern pumpkins are already approaching 20 cents a pound, more than double the usual mark. In Illinois, one of the top five pumpkin-growing states, many growers say they'll be lucky to save 10 to 25 percent of their normal crop. Farmers planted jack-o'-lantern, or ornamental, pumpkins on about 2,130 acres in 1992 and brought in a total of $3.25 million. A torrid summer killed many pumpkin blossoms or ripened crops too early this year. And cold weather is predicted for this weekend, which could finish off what little of the crop remains. Pumpkin farmers across the country have been hit hard this year, especially those in Illinois, Ohio, Pennsylvania, New Jersey, and other Eastern states, according to industry watchers. Gourd-like pumpkins specially grown for pies don't seem to be as affected, probably because many processors grow their own crops and irrigate.