Agree with BOF.
They don't make money on junior days. There is no revenue. Typically they'll give you tickets to (since we're speaking of January) a home basketball game, but everything else is on your dime.
That being said, they will all be there, and they will put on the dog and pony show for your son and the other boys. It's a chance to see what they want you to see, about their campus and their baseball facilities and program. It's also a chance for them to see your son, whom someone must've put them onto, in the flesh and to see what kind of a kid he is. (So make sure he takes the visit seriously, as it is just as easy to make a negative impression as a positive one.)
Where I agree with BOF is, when you're weighing whether to make the 800 mile trip you need to consider whether this school is really on your son's short list. If he's just enjoying all the attention from a lot of schools, then it's time to have a talk with him about a more structured approach to his recruiting. If he's only being pursued by a very few programs, then I wouldn't be quick to pass up this opportunity. If this is his dream school, then YES YES YES you should definitely go.
Decide where you are on that spectrum and you'll have answered your own question about whether going is worth your time and money.