Well... there are so many 5* guardians and their range of usefulness would vary...
I'd say for now, the range that I mentioned,
expect 20-30% devaluation for Berneside and Glaverow guardians except Dominion (eg Ace Stymph bird maybe worth 50-70 STAT-III)
expect 35% devaluation for Bahamut (and 20% for other limited editions) bad types
expect 15-35% increase value for desert type (Sexy Nuada probably worth around 15-20 STAT-III, Ace Serenade around 120-140 STAT-III)
For good type and ace, add 2 STAT-III if full level (or 1 STAT-iii if above level 40), and 2 STAT(III) for every stat maxed (also for enough mp for casting +4 twice)
For decent and bad types, add 1 STAT(III) for each stat maxed, and none for level.
*Remember that bad type stoned will most likely still better than good type clean. But the resell value will be less, as good type will further improve with stoning. If you are new (I consider anyone below level 30 as new) and short on stones, buying bad type with stones will still improve your 4* deck.
This is just a general guide. Expect fluctuations with change in demand and supply. Eg Last week's desert only coli has increased prices for those guardians by 10% or more. And good limited types Barbariccia, Rubicante and Cagnazzo will shoot up in price when it's gone, probably up to 50% more.
I don't even have that much... Hardly any left after stoning the 3* deck
Well, at least there are mods looking over the thread now. Brett123 made a comment there.
Update: here's a dilemma... tt123 who made the STAT(III) offer is actually an RMT trader, selling off seemingly legit 4* and 5*. I am not sure if the STAT(III) are legit and not duped. Question is... is it ok for Negisf to express doubt about the offer? Will the mods take action about tt123? The truth is, he made the offer in STAT(III) and not real money, but the massive stones transfer can get Negisf into trouble too!
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From my trade earlier today...
Jap GC:
Lv 47 Chaotic Phoenix
Lv 13 Sexy Gargantua
Lv 7 Ace Power
2GS plus a bunch of STAT(II) and STAT(III)s
for my Inter GC's Lv 1 Chaotic Behemoth
This may sound like a bad deal, but I have Lv 60 Aces (Phoenix and Gargantua) awaiting rebirth. Ace Power is still pretty decent when reborn, and good to match my Reborn Caim in deck

Hmm... Now I have 9GS in Jap GC... should hunt more now... TGI GS Friday is upon us!
Social battles with improved 4* Aces deck:
IGN: score (positive means how many of my deck survived, negative means how many of yours survived)
FallenOut: -6
Nightwind: -5
funnytone: -5
Last Wolf: -5
Marcus Yap: -2
Ghozt: -2
ORO: +3
Negisf: +5
StitDawg: +8
8788: +9
I noticed that since the giveaway, a lot of teams had improved. Very good indeed.
Time to step up the game. Next time, I shall craft a deck of half 5* and 4*, average stat 3000.