The first was pretty successful. So you would think they would like to stick to what works and possibly build upon it. But this has no bankable names very limited advertising and as you said a 23M less budget.
Could Jack have really cost them 23 Million more in the first movie?
Word has it Black and Sony had some kind of falling out and that's why he didn't return. Same Director Rob Letterman.
Could be one of those things that the studio really wanted to make this movie and I'm sure it has a time frame on the rights. Then when things weren't working out with Black they went ahead with it anyway keeping the director and budget minus Jack. Rushed a script revision and saw that the final product wasn't great and decided not to invest heavy in the marketing.
I think they are replying on Goosebumps as the bankable name. Couple that with a Halloween release, it will probably make a bunch of money. My kid wants to see it, I'm sure millions of others do to. I don't think kids are that discriminating, so why not make a cheap movie and cash in. I didn't think the first one was great, but it was watchable.
It ended up making $93 million on a $35 million budget. That's not bad but the first film made around $150 million so there was a fairly substantial drop from the first film to the second.
From what I've read, Jack Black wanted to go make The House With a Clock in Its Walls instead of doing a Goosebumps sequel, but his schedule opened up later in which allowed him to film a few scenes for Goosebumps 2.