Javascript, 191 bytes

t=prompt(q=[]);s=((t[m="match"](/[!?.]+/g)||q)[l="length"]||1);y=(t[m](/[aeiouy]+/g)||q)[l]-(t[m](/[^aeiou][aeiou][s\s,'.?!]/g)||q)[l]*.33;w=(t.split(/\s+/g))[l];alert(206-1.015*w/s-84.6*y/w)

Javascript, 191 bytes

t=prompt(q=[]);s=((t[m="match"](/[!?.]+/g)||q)[l="length"]||1);y=(t[m](/[aeiouy]+/g)||q)[l]-(t[m](/[^aeiou][aeiou][s\s,'.?!]/g)||q)[l]*.33;w=(t.split(/\s+/g))[l];alert(204-1.015*w/s-84.5*y/w)

First test case gives 112.9 (correct answer is 111.4, off by 1.5 points)

Second test case gives 67.4 (correct answer is 65.1, off by 2.3 points)

Third test case gives 1.7 (correct answer is 3.7, off by 2.0 points)

Javascript, 199 bytes

t=prompt(q=[]);s=((t[m="match"](/[!?.]+/g)||q)[l="length"]||1);y=(t[m](/[aeiouy]+/g)||q)[l]-(t[m](/[^aeiou][aeiou][s\s,'.?!]/g)||q)[l]*.33;w=(t.split(/\s+/g))[l];alert(206.835-1.015*(w/s)-84.6*(y/w))

Results:

The "Green Eggs and Ham" excerpt: 115.7

The "1984" excerpt: 70.1

The excerpt from the Declaration of Independance: 4.8

Javascript, 191 bytes

t=prompt(q=[]);s=((t[m="match"](/[!?.]+/g)||q)[l="length"]||1);y=(t[m](/[aeiouy]+/g)||q)[l]-(t[m](/[^aeiou][aeiou][s\s,'.?!]/g)||q)[l]*.33;w=(t.split(/\s+/g))[l];alert(206-1.015*w/s-84.6*y/w)