For my brides the cost of the trial is included in the bridal rate. I require a 50% deposit to reserve their date, but if they want to have a trial with me before putting down a deposit (I call it a pre-trial), then I only charge $25 but they must to come to me. I let them know that since it is a pre-trial, I will simply apply a standard bridal look on them. They can give me some general input but I don't really let them pick colors and I don't allow them to get into specifics. I basically just want to show them that I am a professional and we can do whatever specific look they want AFTER they have signed a contract with me, since that is what the REAL trial is for, lol.
I haven't had trials for other situations yet. However I don't really see how they would be worth your time (unless its for a photoshoot for your portfolio or something) unless you charge a good portion of what your regular rate is. Trials always take more time and effort, and you should be paid for that!
But whatever you do, do not offer free trials! Have a contract in place first or charge gas money or a kit fee at the very least! Otherwise people will take advantage of you, and you will waste your time and products for nothing. I once heard a story of an MUA who offered free trials, and one time she went out for a trial and it turned out that the lady was heading down to the courthouse right afterwards to get married! She charged for trials from then on, lol! People will try to squeeze everything they can out of you, so be careful!