I built mine to look like my house, using building supplies I had on hand (like extra shingles from my last roof repair, an old cabinet pull I had laying around, etc.) I think the only actual expense was the 4x4 post, the plexiglass for the window, and the hinges.
I have had it for nearly a year and I have filled it with books 3 times. Books just come and go at random, it's kind of amazing. I have it registered at
littlefreelibrary.org and its on their map, but I think my neighborhood has just discovered it and books just kind of show up when it gets low.
I have not had any problems with vandalism and have gotten great feedback and comments (it's a kick to sit on my enclosed front porch, just out of sight, and hear people discover and comment on the library, and look through it).
While I try to avoid Pinterest there are some great Little Free Library boards with inspiration for ideas and even downloadable building plans.