A tailor may offer a discount for multiple orders after the initial pattern has been made, you pay upfront, etc.
Some tailors offer a discount on additional trousers if purchased with a suit, so some tailors do offer discounts; although it may not be as simple as taking a certain percentage of the base price. For example, if there is an upcharge of $40 for a certain cloth from the base price, the tailor may not charge you the difference.
However, do not ask for a discount in your first meeting - get the first pair made, wear at least 3/4 times and assess if it needs any alterations. When you visit the tailor again for further orders, while discussing how many trousers you want made and in what cloth, you could if they offer a discount on paying upfront or if you ordered x amount of trousers.
Be careful though, asking if they offer a discount or promotions is difficult without trying to offend. From your discussion with the tailor, they may mention if you order 4 pair of trousers (for example), pay upfront, they could offer a certain discount; or give an indication if they offer a discount. The issue with asking about a discount, is some people are upfront and expect a discount on a suit or one pair of trousers, or even on a first order.
A little research does help, some tailor websites may mention any promotions or discounts. If the tailor has a promotion on suits with spare trousers, worth mentioning you saw the discount online and ask if they have other promotions.