I just noticed the same thing. Yesterday my SG had around 64K. After logging in just now we have 554K, and I know for a fact that this is not from player activity.
I dare not spend it yet until this is confirmed as not-a-bug. 
Oh, this is on Triumph btw
Check the announcements forum. Your SG just got 20k prestige for each member.
BLAST! Beaten by Positron!
Woohoo! Thanks a bunch for this!
It's a conspiracy!!
Now instead of focusing on reaping the benefits of double XP, I'm going to waste time playing architect in the base.
Thank you devs!
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Beaten by Positron!
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If you've ever been on the RSF or to RV, you'll know that this isn't really out of the ordinary. 
Positron, once again, you rock. AWESOME changes to the game and awesome little bonuses! w00t!
Holy crap! w00t!!!!
Thanks, but damn, am I kicking myself over doing a big clean up of the register a few days ago ... 
Same here, I just kicked three users last night who hadn't logged in for over 60 days.
:-(
Woot! This will really help the new SG we just created. 
50 Fire/Kin Cont
50 Fire/Axe Tank
50 Spine/Inv Scrap
50 Eng/Dev Blast
50 Claw/SR Scrap
50 Emp/Dark Def
50 Eng/Elec Brute
50 Fire/MM Blast
My DeviantArt Page
OMG the other day I counted our sg members (its a small sg for my friends' alts and mine), we had 45 members....x 20k...wow.....woohoo!
Thank you!
Thanks, NC Soft, for closing my favorite game ever without warning and with plenty of life still left in it, and thus relieving me of the burden of EVER wanting to buy, try or even hear about another game from your company. Will my decision make a dent, or persuade them in anyway, shape or form? Nope, obviously not. Don't care. NC Soft is dead to me. ~ PsyFox
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Check the announcements forum. Your SG just got 20k prestige for each member.
BLAST! Beaten by Positron!
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CRAP...I deleted like 5 inactives yesterday! lol
I deleted 3 also! But I don't care! The 900k or so that I needed in order to complete my base as desired is now in the bank.
Thanks devs!
"...freedom isn't a commodity to compromise." -- Captain America, New Avengers #21
Guide to Base Teleporters
this is totally awesome, thanks devs!
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Same here, I just kicked three users last night who hadn't logged in for over 60 days.
:-(
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Did this myself a few weeks ago. 6 characters x 20k prestige = sad panda
I should have made the innactive character time limit 60 days vice 45
(mild sarcasm) Woo Hoo! Our rent just went up! (/mild sarcasm)
Sorry, had to tease a bit there. Heh, the SG I'm in only has 8 players but is nearly full with everyone's alts. We just raked in over 1.4M prestige
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Not a bug (and not 5%):
Cricket's Announcement
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so the radioactive little bugger does snoop around here, maybe he even reads this stuff.....wow
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It's a conspiracy!!
Now instead of focusing on reaping the benefits of double XP, I'm going to waste time playing architect in the base.
Thank you devs!
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Amen Brotha.
I already spent the 1.2 million prestige our SG got. Upgraded the control room to a secure control room.
Sadly, you can't tell the difference. However, the next raid we have... We got defenses now baby!!!
And though the weekend long lag fest will certainly piss me off... Atleast I have my 1.2 million prestige to show for it.
The funny part will be when rent comes due. Haha.
First thing I thought of..between this spike and the spike from the extra prestige from the weekend there may be some hurting SG/VGs if they spend it all before rent comes due again.
lol yep im totaly kicking my self now why cuz i just cleaned are supergroup roster today befor the matinance got ride of 15 people (((((((( i think im going to go cry in a corrner now.
I'm just overjoyed! That plot upgrade is looming ever closer over the horizon! Thanks Cryptic!
P.S. Ok, announcement about teleporters for COH, new base items to help small groups, double XP weekend, free prestige bonus per member.... Why do I feel like we're being setup for something?.... 
Glee!
Added: Medical room, Enh and Insp storage. The Arcanos 2-player+some-alts-PvE-oriented-base is now complete. ^_^
Rent is going to be hurty, but I just paid it (power went off today) so hopefully we won't have to worry about that for a bit.
From the Champion server:
It appears that we are experiencing a 5% bonus to all SG's total value.
The Blackguards were valued at 20,000,000 total with approx. 2.8M banked, and now are valued at approx. 21,000,000 total with 3.8 banked. Other SGs are reporting different bonuses, and it appears that the bonus was approx. 5%.
Confirmations from other servers? Explanation? No, screw the explanation. IT'S BASE BUILDIN' TIME!