If we look at the AKB related recent solo releases we have the following sales:
Iwasa Misaki – Mujin Eki: 23,773
Ono Erena – Erepyon: 25,740
<gap>
Sashihara Rino - Ikuji Nashi Masquerade: 68,403
Itano Tomomi - 10nen go no Kimi e: 72,911
Watanabe Mayu – Hikarumonotachi: 91,907
Based on Kasai's senbatsu ranking it makes sense she's in the gap between Erena and Sasshi. But yeah, I would have figured she'd end up in the 50K area at least. 
Can't complain too much though. Looking at other solo idol artists, a lot of them struggle to break 5K even with daily events.
I do wonder what the effect of the graduation announcement is. Did it help or hurt the sales...
Weekly numbers out - 3rd place with 32,582 copies sold.With the interest in the sales numbers of AKB48 Stages Studio Recordings, there's data for the other AKB48 singles and soloists as at January 7, 2013:
1/7付 ウィークリー
**3位//32,582/河西智美「まさか」
**4位//30,683/NMB48「北川謙二」(累計398,155)
**6位//20,549/乃木坂46「制服のマネキン」(253,510)
**8位//14,547/AKB48「永遠プレッシャー」(1,165,527)
*12位//*4,828/AKB48「UZA」(1,228,003)
*46位//*1,345/AKB48「ギンガムチェック」(1,309,352)
*51位//*1,173/渡辺麻友「ヒカルものたち」(117,777)
*66位//**,921/NMB48「ヴァージニティー」(393,345)
*82位//**,716/SKE48「キスだって左利き」(589,959)
126位//**,463/乃木坂46「走れBycicle」(241,787)
171位//**,356/NMB48「ナギイチ」(449,454)
176位//**,346/指原莉乃 with アンリレ「意気地なしマスカレード」(85,365)
178位//**,342/乃木坂46「おいでシャンプー」(221,535)
181位//**,329/AKB48「真夏のSounds good!」(1,820,385)
184位//**,321/AKB48「ヘビーローテーション」(880,164)
Capped by
tomomioshiI re uploaded to
imgur