The big thing regarding pricing - Tyre model, size, casing, compound, etc... will effect pricing. Then availability will come and go, prices will fluctuate regularly, and then there's occasionally specials that might nab you a better deal at sites that are typically a more expensive option. Only you know what you're looking for, only you can decide when exactly you're going to buy it, so the only way to know what's cheapest in that moment is to search (and keep an eye on the Bargain Buys/Discount Code threads).