You could use "at a discounted price", "at a discount", "with a discount", "on sale", "at sale pricing" and "on clearance."
Discount is a general term, but is also used for special deals that only you can get, like educational discounts for being a student, or a discount from the store for shopping there for so long. Sale is for discounts anyone could walk in and get. (Sale can also just mean that it is available to be bought, so be careful for confusion.) Clearance is for goods that the shopkeeper doesn't want in stock anymore, and so the shopkeeper gives them a big discount so they sell quickly until they're all gone, even if it means losing money compared to the price they paid.
I don't really like your second sentence, "with a discounted price" doesn't quite come together. It's just a little off. You're also right about "in", I can't think of a single phrase that would work with.