Wow, this answer has spurred up controversy...Here's a redrafting and clarification of why this answer is not a generalization.
I have provided extensive citation (from Princeton.edu, a Vanity Fair George Lucas Interview, A Hitchcock interview, Merriam-Webster and others) in the links below to support my answer (I have researched this answer...it's not just my opinion):
Film makers that avoid showing the value of money show that they know about film making and film history, and by so doing, are paying homage to Hitchcockian theroy of MacGuffin & respect to their craft.
This answer relies on clearly stating and showing why this situation is a MacGuffin, and why it is crucial to understand the rationale filmmakers use to determine whether or not to show the true nature of the MacGuffin.
Why a check with an undisclosed amount has to be and can only be a MacGuffin
The check with undisclosed amount used to entice a character into action is a variation of a cliche. As a cliche, this situation occurs in the first act. The protagonist receiving money is a plot device that's referred to as a MacGuffin. In the story a check handed to him could be anything. It could be his fathers gold watch that was smuggled out of 'Nam. It could be plans for building the Death Star. It only needs to be something that will prompt the character to initiate an action. It doesn't need to be a check. But if it is, it doesn't need to have an amount - the amount is only important to the character (not the audience). Since it is something that is shown (a check), but it's true nature is not revealed (we don't know the amount), it can only - and has to be a MacGuffin (the briefcase in Pulp Fiction and Ronin...in a jewel heist movie it's the necklace, in a spy movie it's the papers). This is the definition of MacGuffin and the reason the term exists. If it was important to the plot for the amount to be shown, it wouldn't be a MacGuffin. Since it's not shown, it's not important to the plot - it has to (and can only) be a MacGuffin
In this instance the payment gets the protagonist to start the ball rolling, after that the plot develops on it's own without further need of catalyst.
How and Why film makers deal with the decision of revealing or not revealing the true nature of the MacGuffin:
Hitchcock said that what the MacGuffin is doesn't matter to the audience. It could be anything.
George Lucas says the audience should care about the MacGuffin (in Star Wars, it's R2-D2; everybody is looking for R2-D2) as much as they do about the hero and villain. Then he does this.
Rationale Film makers use to determine to show, or not to show the true nature of the MacGuffin
The film maker can choose to:
show an amount and tell viewers and critics that he is from the George Lucas school of thought (the guy that brought you Jar-Jar Binks and Indiana Jones and the Kingdom of the Crystal Skull)
or
not show an amount and show critics and viewers he's from the Alfred "True King of Filmmaking" Hitchcock school of thought.
Not a tough choice to make. It has to be one of these two rationalizations because Hitchcock and Lucas are on record about their feelings of the MacGuffin and this is taught on day one of Film History 101. All directors know about the MacGuffin and the relevance to film history.
In conclusion...The answer to the question:
Film makers that avoid showing the value of money show that they know about film making and film history, and by so doing, are paying homage to Hitchcockian theroy of MacGuffin & respect to their craft.