Great points made in this thread.
I would add this. Stop thinking about cameos, they're no good. Actors don't like doing them, their agents don't like cameos since they cheapen their client's name value and image, and buyers and distributors don't like cameos, they want name actors to have decent sized roles because in the end, the customer who decides to buy or rent a movie because it has their favorite actor in it, wants to see that actor to be in the movie for a considerable length of time not just a quick cameo.
Agents, buyers and movie watchers have become wiser. The old "let's hire a name actor for a day so we only have to pay them a day's salary and we can put their face on the poster to sell the movie" is a no go, doesn't work.
The name actor and their reps know that the only reason you want to hire them is so that you can use their name to sell the movie, therefore they want to be paid accordingly, and want decent sized roles.
That's not to say that serious actors don't do one-day gigs, because they do, it all depends on the project, the role, the money etc.
You just don't want to hire someone like Eric Roberts who everyone knows does a hundred movies a year and whose name is pretty much worthless now.
That being said, three days is not a cameo, and it's not a bad gig for a name actor. For $25,000 you can't afford an A-lister, but you can get a lower level B-lister probably.
Needless to say the production needs to be SAG, and there will be additional fees to consider like has been mentioned in this thread before.
Before you pay anyone that kind of money, just make sure that they are a name that matters, you want someone who can get you distribution.