In "One Thousand Dollars," what is the meaning of the lines "If it had been $10,000, a man might celebrate with a lot of fireworks. Even $50 should have been less trouble" and "That is, part of it goes to the man who invents a new bacillus and the rest to establish a hospital for doing away with it again"?

The first quote is a reference to Gillian's inheritance. His deceased uncle has bequeathed him the sum of a thousand dollars. Gillian maintains that a larger or smaller sum would have been more to his liking. He feels that he would have put a larger sum of money to better use. He uses the word "fireworks" as a metaphor to characterize how much he might have enjoyed himself if his inheritance had been larger.
Also, with a bigger sum, the work of documenting all his expenditures would have been worth the time it took. Conversely, a smaller sum means that he will have to do less work in documenting his expenditures. This is why he mentions that "Even fifty dollars should have been less trouble." Gillian considers a thousand dollars an awkward sum; presumably, he can't spend lavishly enough to justify the time it will take to document his expenditures.
The second quote is quite interesting. Gillian tells Old Bryson that his uncle has essentially left the majority of his wealth to some scientist who will invent a new "bacillus" and "the rest to establish a hospital for doing away with it again." A bacillus is basically bacteria. A scientist may discover a new type of bacteria and name it (this is presumably the "inventing" Gillian is referring to). Once the scientist names it, he will also have to discover a way to be rid of the bacteria, especially if it causes illness in human beings (this is the part about "doing away with it again").
As an example, Filippo Pacini and later, Robert Koch, discovered the cholera bacillus, which was named Vibrio Cholerae. Meanwhile, chlorine has been discovered to kill waterborne pathogens, including those which cause cholera and dysentery (please refer to the links below for more information).
Interesting, isn't it? Essentially, Gillian's uncle has left the bulk of his wealth to science, specifically to support the discovery of bacteria and to finance research into the cures for illnesses associated with such pathogens.
http://www.ph.ucla.edu/epi/snow/firstdiscoveredcholera.html