I think that what is said above is correct, but also, they are starting to reach the cost of breeding, if you see what I mean.
Previously there has been a pretty much 2 tier system - those bred for performance, where stallion fees, stud livery fees, vets, scans, swabs, transport, vaccination etc are all done as they should be. This isn't cheap - I've bred a foal this year from my daughter's pony, and all in all he's cost probably over £1.3k to get to his current 4 months of age. Bearing in mind I have my own land, so no livery fees, and his dam is an easy to keep, native good doer. Once I add on gelding, feed, vaccinations, registration, worming and farriery for him, at weaning he'll have cost over £2K - and as we do it ourselves, there are no staff costs (that at a stud, there would be). Previously foals of his type would sell for less than half of that, sometimes because people don't think about the cost before they go into it, sometimes because they can't sell for more, but more often because the market is flooded with foals who are the result of someone just running a bunch of iffy mares with a stallion in their field, no vets, no vaccinations, no worming, scans etc. Because of this many good breeders have stopped breeding - they can't break even, so unless breeding top performance horses which could go on to compete as eventers / SJ etc, they can't get the money. That's meant that over the last decade of so there is a big split between the cost and quality of the top and bottom, with not many just nice riding horses in the middle.
Suddenly, lots of people want a horse, there aren't as many about (unless you want a gypsy cob) so they are actually making what it costs to breed, break and produce.
We bred our foal as we want to keep him for ourselves, knowing that we wouldn't be able to sell for what he cost, happy with that, hope he makes a decent enough size, and is at least half the horse he could be!
I think the bigger issue with many for sale now isn't so much the cost, but is the number of horses being sold that are not what they are advertised as - lame, with vices, or just a bit less suitable to a novice rider than they say. Lots being sold unseen, and also many people who are looking to buy who are unrealistic about their experience and ability. A friend was selling a competition pony, advertised as not suitable for a novice (he's a sharp but very able 138 BS pony who knows his job) and she had no end of people whose child had been at riding school for 6 months, had learn to rise to the trot but wanted to do showjumping, who rang to enquire.
Anyway, good luck with your search, be prepared to wait, I have no doubt that there will be lots for sale as the weather gets worse, the horses get fresher and the less experienced owners realise the reality of horse ownership in a cold wet winter, with few competitions and limited ability to get lessons. However, some of those horses will need a certain amount of retraining, to return themselves to their previously nice natures.