For me, that decision would be based largely on what else is in the panel.
In my case I have dual VOR's with one of the being LOC/GS/MB. This gives me what I need to do an ILS. That's why I hassled around getting my MB working in preparation for my instrument training. If I had a 430 or 530, it would be a different decision.
I now have the means to do a precision approach for my checkride.
All that said, depending on what your needs and other equipment, $100 extra sounds like a good deal to have it installed, integrated and good to go. It's kind of comforting sometimes to hear that irritating sound go off.
BTW, for $100 I don't see how they can even add the antenna for that much. Are they going to split the MB signal off of a NAV antenna? If so, that will give you only marginal signal strength. Over the holidays, I changed splitters to get rid of an attempt to split the MB signal off the antenna. I added a VERY UGLY MB antenna under the belly and the antenna alone cost over $100.
Also BTW, that looks like a nice audio panel. Even nicer than my 80's era Narco.