Hey !
I didn't personally go to either Mannheim or WHU but I've heard good things about them and they were part of the schools that I applied to.
Our friend (previous comment) already explained what ROI is, but looking at most base salaries of MiM graduates these days you shouldn't worry too much about that. After getting your MiM, most people will look to take jobs as project managers, strategy consultants(preferably if you have a little background experience in business), marketing managers, social media managers and so forth ...
If you don't want to provide a GMAT score you should be asking yourselves other questions that can make up for this, such as: what valuable work experience do you have? Do you have an international experience? What is your knowledge of German (especially important for post Mannheim or WHU prospects if you wish to stay in Germany)?
And if you are looking for a consultancy, I highly recommend EssayAdmissions. Their consultants were super professional, always looking to help and have a lot of personal advice to offer you as they're all alumni of top B-schools. Thanks to them I got into the MiM program at HEC, but they also offer admission consulting to the schools you're interested in. I've heard good things about the other ones though.
Anyways, hope this post helped you a little bit

if you have any other questions feel free to send me a private message !