Yes, that price pattern seems to be right, except that they have reduced a 6300 to 4800 in the mix (Skarner 4/2013). I looked up the past cases and their current price. (Table 2)
It does seem a 3150 to 1350 is most probable, unless they've been recently reduced already (Table 1).
Table 1
Current 3150 IP Champs
A
Akali
Anivia
B
Blitzcrank
C
Corki
G
Galio
Gangplank
Gragas
H
Heimerdinger
K
Karma
K cont.
Karthus
Kassadin
Katarina
L
LeBlanc
Lux
M
Miss Fortune
Mordekaiser
N
Nidalee
O
Olaf
P
Pantheon
S
Shaco
Shen
Shyvana
Sona
T
Twitch
U
Urgot
Table 2
Date Released Champion Price Reduced for...
12/20/2012 Price Reduction Plan Kick Off Ezreal, Vladimir, Renekton, Nocturne, Lee Sin, Brand
1/23/2013 Thresh Release Yorick + Udyr 1350
3/1/2013 Quinn Release Leona
3/28/2013 Zac Release Wukong
4/30/2013 Lissandra Release Warwick + Skarner 4800
6/13/2013 Aatrox Release Talon
8/22/2013 Lucian Release Riven
10/10/2013 Jinx Release Xerath + Miss Fortune 3150
12/13/2013 Yasuo Release Graves
2/27/2014 Vel'Koz Release Shyvana
5/12/2014 Braum Release Fizz + Xin Zhao 1350
8/13/2014 Gnar Release Volibear
9/16/2014 Azir Release Ahri
11/20/2014 Kalista Release Viktor + Amumu 450
12/11/2014 Rek'Sai Release Sejuani
3/12/2015 Bard Release Ziggs
5/28/2015 Ekko Release Nautilus + Shyvana 3150
9/7/2015 Tahm Kench Release Fiora
10/14/2015 Kindred Release Lulu
11/24/2015 Illaoi Release Hecarim + Rammus 1350
2/1/2016 Jhin Release Varus
3/24/2016 Aurelion Sol Release Darius
5/18/2016 Taliyah Release Draven + Singed 450
8/9/2016 Kled Release Jayce
10/5/2016 Ivern Release Zyra
12/12/2016 Camille Release Diana + Galio 3150
TBA Champion 135 Release Rengar
TBA Champion 136 Release Syndra
TBA Champion 137 Release Kha'Zix + a 4800 IP or lower Champion