I have a second soundcloud page that houses a number of before and after mixes.
But does it say this on your advertising?
I would get some direct links up on your page, perhaps with a little bit of "geekery" wording about what you did to the mixes and why it is a good idea to pay you to work your magic
If you are doing this as a business you really need to "sell your wares" and make it as easy as possible for potential customers to see exactly what you can do - especially if they can relate it to their situation
I am guessing your biggest potential market is going to be those that can record a rough "guitar & vocal" (eg on an ipad / iPhone) that do not know how to take their song to the next level
I would think those that are going down the home production route will want to do their own production/mixing (which is why I think there could be an opportunity with the "mastering" approach but I agree this is a specialism and it is refreshing to see you recognise that as well)
I think it would be a good selling point to show a "before & after" of a rough demo that has been remixed and "produced" by you in to something awesome
These are just some "off the top of my head" comments to try and help