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When it was first launched, it was billed that Dynamic Pricing would offer passengers the cheapest seats if purchased at the time of the main ticket pricing. It would not be cheaper than at that point. And certainly on short haul it appear that if a seat was £7 during ticket purchase, it would rise to £9 or so immediately thereafter.
That said, if the airline wanted to maximise income then logically a differential (lower) price point would make sense at some stage, something that Ryanair seems particularly adept at manipulating.
I haven't seen a lot of evidence on the details of how this process has actually worked, but I'd suggest if you are definitely going to purchase a seat you best do at the time of booking.