It is going to be hard.
Manufacturers will be phasing out the rx 470/480 and focus on releasing the 570/580 series instead.
Don't know if the 4xx series will be phased out completely.
There will be no more 400 series as 500 is a refresh and replacement of all models. Its very similar and mostly being done just to allow cheaper production costs for AMD, eventually customers see some benefits but for now its only slightly faster at stock clocks.
So after the RX5xx is out, the 4xx will be much cheaper?
No 480 or similar will be no cheaper as its essentially the same card. Maybe you can save 10 dollars here or there but I dont think we are talking much. Nobody will switch from 480 to 580 for example as there is no advantage.
We've already seen some offers for 480 at times and that is all the effect possible really. So far all I've seen people comment is that the 580 is more expensive, but the 480 is no cheaper because of that. 580 will match Vega price on release I think, as you can double them up its possibly a rival in performance so they will not make 580 that cheap these few months.
If 580 is no cheaper then 480 not cheaper either, imo second hand prices stay high due to miners filling any demand gap