I have no idea about the product quality, do your own research.
But on paper this might be the cheapest Dual SIM android phablet with respectable set of specs for the price.
Byond PI Phablet at Rs 7,122 >> http://www.ebay.in/itm/271270895125?_trksid=p3984.m1438.l2649&ssPageName=STRK%3AMEWAX%3AIT#ht_3126wt_1163
Official Webpage Link >> http://www.byondtech.com/byond/smartphones/phablet/pi/#
5.3 Inch qHD IPS Display, Resolution: 960 x 540 pixels
1GHz Dual Core Processor
Android 4.1.1 Jelly Bean, Multi-window interface
1GB RAM, Internal Memory 4GB, Upto 32GB Micro SD
Rear Camera: 8MP Front Camera: 1.3MP
Dual SIM(GSM+GSM),Dual standby/single active, 3G
2000 mAh battery
EDIT: There is another model with even better specs for Rs 1,132 More...
Byond Phablet P2 at Rs 8,254 >> http://www.ebay.in/itm/Brand-New-Byond-Phablet-P2-5-7-Screen-8-mp-Dual-Sim-1ghz-dual-Core-Processor-/300959643220?pt=IN_Mobile_Phones&hash=item461297b654&_uhb=1#ht_2898wt_1031
Official Webpage Link >> http://www.byondtech.com/byond/smartphones/phablet/pii/#
5.72 Inch HD IPS Resolution: 1280 x 720 pixels
1GHz Dual Core Processor
Android 4.1.1 Jelly Bean
1GB RAM, Internal Memory 4GB, Upto 32GB Micro SD
Rear Camera: 8MP Front Camera: 3MP
Dual SIM(GSM+GSM)
2500 mAh battery
Complimentary – Screen Guard, Flip cover, Power Bank