Its bees, so its complicated in detail.
Firstly it depends on what you want.
For example, Bee Hive Supplies (makers of better, and more expensive, than average poly National and 14x12 kit) never seem to have a sale on their polyhives. But, it might be different if you dropped in and haggled for a while over any 'dusty' stock they might have.
You won't get sale prices on foundation - just quantity discounts, so get some pals together (or involve your association) and bulk-buy.
Thornes first quality frames and hives aren't normally discounted. But they routinely offer 'seconds' at sale time. We noted last year that face-to-face sales have marginally (not massively) cheaper prices than the New Year online sale, so if you are going to the National Honey Show, the Convention, or live near one of their shops, that's as cheap as it gets. But if you order £100+ in the online sale (very easily done) you get free delivery and don't use any petrol/diesel.
Sometimes Thornes have a few first quality hive parts (presumably ex-demo or somesuch) in their sales - but you can't predict what. Whereas you can be pretty confident of 50-packs of frame seconds being about the cost of 20 primes. This January there was only one (I think) extractor motor at half price. You'll find similar one-offs (similarly unpredictably) at their shop and exhibition sales.
Paynes sell complete polyhives at a slight discount in October, but you'll have to wait until January if you just need more supers and another roof, and sale discount really matters to you.
Jar manufacturers seem to often have specials at the exhibitions. But Association bulk-buying (and bulk delivery) is usually the best bet.
AgriNova sometimes have ex-demo extractors and customer trade-ins (against upgrades). If you were looking for something particular, it is well worth contacting the vendors and asking them to keep your name on file as looking for a deal.
Sorry, no simple answer. But then, its bees.
// And if you hear of any bargains, let us know!