First of all, don't use
sudo su. It's not wrong, or dangerous or anything, it's just inelegant and pointless. You are running two separate programs to do a job easily handled by one. If you want to start a shell as root,
sudo can do it for you.
If you want to start a login shell (that's what
sudo su -) does, use
sudo -i. That, however, since it starts a login shell, will take you to root's home directory by default.
To start a regular, non-login shell, you can use
sudo -s. That will start a root shell for you and keep you in the directory you ran it from:
terdon@tpad ~ $ pwd
terdon@tpad ~ $ sudo -i ## changes directory
[root@tpad ~]# pwd
[root@tpad ~]# logout
terdon@tpad ~ $ sudo -s ## doesn't change directory
[root@tpad terdon]# pwd
So, the way to start a root shell and stay in the same directory is