Example 1
Fifty dollars is a lot of money for a t-shirt.
It sure is. And is is the correct verb here since we're dealing with a quantity of money. Think of it as one, singular wad of sweaty cash.
Example 2
Two hours seems to last forever when you're sitting through an Adam Sandler movie.
Unless that movie is Happy Gilmore of course. Since we're measuring time here, the singular verb seems is the right call.
Example 3
93,000,000 miles is the distance from the earth to the sun.
That's 150 kilometers, if you're a fan of the metric system. No matter what units you use to measure distance, you're going to want to use a singular verb.