When Herbert suggests that his father wish for two hundred pounds in order to pay off the mortgage on their house, the reader senses that such a modest wish could only cause a small amount of trouble if it caused any at all. This is appropriate because the suspense and terror build up throughout the story.
Mr. White would have wished for something greater if he hadn't felt a little frightened by all the other foreshadowing that led up to his making his wish. He is not really wishing for a small sum because he doesn't need anything more; he is wishing for a small sum mainly because he is terrified of the possible consequences. There has been nothing but ominous foreshadowing up to this point. Sergeant-Major Morris has warned them repeatedly against using the monkey's paw and has even tried to burn it in the fireplace. He also told them:
The first man had his three wishes, yes," was the reply. "I don't know what the first two were, but the third was for death. That's how I got the paw."
When asked how to use the paw, the sergeant-major tells White:
"Hold it up in your right hand and wish aloud, but I warn you of the consequences."
Mr. White actually paid his friend a small sum of money for the paw. He wants to have it and to use it, but he is afraid of it (just as the reader would be). He has to be persuaded to make his first wish. He is reluctant even to ask for a relatively small sum of money because he is venturing into the unknown and tempting fate. The reader's feelings are that he would like to see if the wish will be fulfilled but dreads reading that something terrible happens to this nice little family.