One could make a system for autogenerating simple packages, but I have seen none. All of the Debian packages which I know about have hand-built rules files. Aside from that, most of the files in the debian subdirectory look much the same (with scriptable differences) across packages.
For what it's worth, I have built 48 different packages for my own work, and provide the corresponding scripts in the source-tarballs, but in a different subdirectory to avoid conflict with Debian's packagers. Debian does not distribute their build-scripts within the source tarballs, because their policy is to distribute patches against a pristine (unmodified) upstream source. So likely the only sources where you would see the two combined are from developers who build their own packages.
You might be able to adapt the scripts from a given Debian package (though some have odd quirks). To start, you need to find the package scripts. I usually go to the bug-reporting page
https://www.debian.org/Bugs/ and (not all packages are named consistently), search for bugs on that package, e.g., nginxenter link description here. On that page, is a pointer to the package page. Within that, there are links pointing to the package as of different releases, e.g., jessie (stable), and following that link takes you to a page on which there are (on the extreme right) links to
- [nginx_1.6.2.orig.tar.gz] (pristine source)
- [nginx_1.6.2-5.debian.tar.xz] (Debian scripts)
Debian's build-system assumes those pieces are available separately; for my own builds I wrote a script which assumes that my scripts are in the different location referred to. For either solution, you may have additional reading to do.