The cheapest horses, are usually crossbreds or irresponsibly bred purebreds, which are a conformation nightmare and have had no training or decent care and if a male, is ungelded. Such horses are often dumped or given away free.
Next would be purebreds of any breed, which are to be found in rescues or auctions. In the last few years, some very nice horses and many with papers, have been found in auctions.
After that I think, it would be plentiful breeds in a given area or country. What is common and cheap in one country, could command a larger price where they are uncommon and have a following.
A good horse in any breed, which has been handled and trained, could command a price many times more than the same horse with no training. A stallion who has made a name for himself in most breeds, with offspring to prove his worth, can command a high price.
Wealthy farms with a history of producing excellent stock and an enormous advertising budget, will usually ask and get, more for their stock.
Certain Warmbloods, have usually fetched quite high prices. And by 'Warmblood', I don't mean any old light horse x draft, which many seem to immediately think are Warmbloods.
So most of the time, you can't just go by breed. In every breed, you will find very expensive, less so and bargains. And like everything, it is often a case of supply and demand.