The iMac Mid 2011's SATA 3.0 interface allows for an installation of an SSD much faster than that of the 2.0. You can add an SSD via BTO, but as you know, it will cost much more than if you install one on a standard iMac. The problem with this, though, is that the standard model does not have a built-in mounter and therefore impossible to install an SSD just by itself. (Other problems include: You cannot replace the HD with the SSD, because the fan will be too loud; even if you replace the superdrive (SATA 2.0) with the SSD, the speed of the SATA 3.0 will not be generated; the SATA ports on the logic board is male so modifications are needed with the installing side of the SSD; etc.)
At Your Mac Store, we sell an SSD Installation Kit for iMac Mid 2011.
It comes with an installation manual copyrighted by us.
Internal SSD Installation Kit for iMac 27" Mid 2011, $99.95
We are also a reseller of SOLIDATA 2.5" SSD's, which will be handy to have together with your installation kit. Check it out here: http://www.yourmacstore.com/2-5-sata-ssd/