ssd's are dropping in price all the time, a 256gb drive will set you back about £150.
Apple seem to be getting wise to this fact so they are making it harder and harder to upgrade stuff. To upgrage a hard drive there is kit for the 2011 21.5-inch iMac, which sells for $58 directly from OWC. It includes an 11-piece tool kit, microfiber cleaning cloth, the necessary data and power cables, two suction cups for the iMac glass screen removal, and a thermal-safe adhesive mounting set. That just sounds like trouble to me.