I just put Akrapovic's on my M1100. Look good, sound cool, and with the DB Buster installed, requires no re-mapping. New they are around $1,400. I got mine used for $500 and they look brand new! Shop well my friend, there are deals to be had if you're patient.
You can find termignoni's still but they cost probably about the same as your bike is worth. Mivv's are decent, I just recently got a set of these Dominator pipes out of Poland. I haven't installed them yet but they sound good on other bikes I've heard. Here are all of their options for the Monster 750.