I can't think of any mistake I made by trying to be cheap and having it end up being more costly.
However, I have the opposite story.
When I was in Dubrovnik in 2014, I wanted to take a day trip to Montenegro. But all the "tours" seemed rigid and didn't allow enough time in Kotor. Plus I really wanted to get someone to drive me up to Cetinje, with a stop in Njeguški for the pršut, and I had no interest in visiting the Church of Our Lady of the Rocks. So in spite of the cost (I was traveling solo), I hired a private guide, told him exactly what I wanted to do, and he gave me what I thought was a fair price. I don't remember exactly, but it was in the vicinity of $300 - $350. I told him if he found other travelers who might like to join the tour and split the cost, he should feel free to include them.
The day before the tour, I got an email from him telling me that a family of American tourists would be joining, and my cost would be just $50. I was very happy about that until I learned, when the tour began, that their itinerary was completely different from the one he and I had agreed to. Pretty much their only agenda was to check off another country. We stopped in Perast and took the little ferry to Our Lady of the Rocks. Then when we got to Kotor, we had just an hour. And then we headed back, stopping at a restaurant in some village in southern Croatia for dinner (where I got to dine with this group of strangers who were actually quite pleasant and did their best to include me, but were just American tourists after all, so this was hardly the way I would have wanted to spend my time). I was so disappointed.
The guide's name was Pepo Klaic. Recommended in the RS guidebook. And mine is not the only bad experience with him. (https://community.ricksteves.com/travel-forum/day-tours/pepo-klaic-dubrovnikshoretrip-com-day-trips-negative-review).
So sometimes the opposite happens. You are willing to bite the bullet for something expensive and customized to your interest, and all you get is the economy package.