How much disk space does a list of files use

You can use tail to cut the last line (the total) from the output of du:

du -c *.sql | tail -n 1

There seems to be no way to make du itself report just the total of a set of files.

What doesn't work from your example?  Do you want a sum?  man du shows that the -c option provides a sum of usage:

du -sc *.sql

You may also like the -h or -k arguments.

Your question is very ambiguous but I suspect you are looking for the -c flag to produce a total.

du -c *.sql