Is the mini van trip from Paris to Giverny to Auvers-sur-Oise worth it on a Sunday in early May.?
most probably not if you mean May 1st. See other posts about May 1.
Which mini van trip?
I guess it depends on how much it costs, how much you want to see those two places, and what the weather turns out to be that day!
I would think it would be a convenient way to fit in the two sites, which otherwise probably couldn't be done in the same way. And it also takes the guesswork out of (for going to Giverny) finding the right train station, finding the connecting shuttle bus, then doing the same in reverse.
You are paying for someone else to worry about the logistics. Your time is worth money! You are giving up a little bit of your own ability to decide how long to stay, etc.