I'm not exactly 100% sure of the solubility of MXE in different liquids, but I do remember it taking a long time to dissolve when I tried to put some in water for plugging, this could have just been a cut that wasn't soluble in water. Can't be sure.
I have used propylene glycol for preparing solutions of other drugs that have had trouble dissolving in water, it is such a cheap solvent too that it would probably be your best bet for the 50mg/ml goal that you are looking to get.