You're losing a small amount of performance by not using dual-channel RAM, but if you use up to that 10GB of RAM then you'll far outweigh the performance hit with the benefit of more RAM. I honestly wouldn't worry about it, or if you are, find a cheap 8GB stick instead of buying two 4GB sticks.
The 2012 mini uses DDR3-12800 RAM and you can find oodles of it all over the web used and cheap.