Sorry to state the obvious but it depends upon the bloodlines, age, stallion potential and whether the owner actually wants to sell or not/
There is a 4 year old Section D WPCS Licenced stallion for sale in H&H this week for about £6500 and that sounds about right to me (would be more if he was also SPSS graded though) but the same type of colt could possibly be bought for as little as £1000 as a 2 year old -- even if still a good stallion prospect -- if the owner really needed to sell.
Also if you get two bidders at an auction both wanting the same colt then prices can really fly(possibly up to ca £10,000 for a top winner in hand).