I don't usually offer a discount for mobile, because of the time and consumables. I charge a little more per treatment, £5 if within 3 miles, £10 if further. You could offer a small discount if they planned to become regular, but if they don't ask, they may be happy enough with the prices.
I've become a little cynical about prices, having made a discount of £5 a treatment with one client, who then wanted a further £5 discount, which was agreed for a block booking, to be taken within 3 months, but he came back and tried to maintain the discount over 6 months, then I found out he was a property investor with A LOT of houses on his books, so he was hardly strapped. The ones who push for discounts are usually the ones who have the money (and that is why).