It was my understanding that they are making all HGSS (and it might be B&W too) era packs available in the shop soon, (and forever... not just a week here or there) which will allow players to buy the packs to obtain the cards, instead of HAVING to use tournaments or have codes. (which made it hard to get into for some) so these people are speculating that because people will be able to obtain them for coins, the price will drop... They are probably right... but how far is really the question. For example, FA Cynthia and Lele-GX are holding value and people are able to buy UltraPrism/GUR packs in the store.... IMO it could go a couple ways..
1. more people start playing legacy, and the value of the tradable legacy cards rises.. because more people are playing/collecting legacy... or
2. With more people playing legacy, they instead decide that attempting to obtain cards like Celebi prime via coins/store is the better way to get the card/s they need.
The current price drop is speculation to the latter. Now its up to you to decide if its currently a buy or sell opportunity at 40-45 or not. I personally think it has some room to drop more as people hear about this change in HGSS pack availability. What actually happens after these packs are made readily available is anyone's guess. More than once has popularity taken a good card and made it skyrocket, only to watch it drop, then skyrocket again. (I think the DEX FA Darkrai-EX is one of the best examples of the ups and downs) Assuming you arent sitting on 45 Celebi Primes (or Computer Searches? LOL) you dont have much to worry about, or can expect to pick up a great card at a cheaper price compared to the recent past pricing