prices seem to be higher after the draft due to all the new young players available. it makes more sense too to sign after the draft especially if you are looking to train a player. you have a whole season to do so as opposed to wasting this season which is pretty much over.
I think you can find a good deal on an 18yo turning 19 right now.. and they would likely be more developed than a 19yo draftee (which will litter the transfer list in a week or so)....
But the transfer market doesnt follow a strict set of rules.... the perception of certain weeks / times being more expensive than other seems to stick with some managers.... i simply recommend doing some homework yourself before committing.. its part of the game!