Is it possible to install an SSD and/or another hard drive after purchase in a 27" iMac?
On the immediate downsides - You will need special tools and be handling some delicate parts. Also, there is concern that the temperature sensing since the 2009 model iMac causes the fans to run faster with non-apple drives.

If you really want to get dust between the LCD and the front glass on a brand new mac and take the chance of ripping a tiny ribbon cable - just do some research at ifixit.com or talk with a local tech that works on new iMacs a lot. Do also research fan control and know if you are not planning on an Apple specification drive to install.

My guess is Apple SSD are expensive because the parts are cherry picked to be the best of a lot (more so than the SSD consumers buy) and perhaps loaded with custom firmware and generally have temp sensors specific to the mac. I could be totally wrong, but even if you feel the drives are overpriced, Apple installing any SSD is better than a consumer due to the labor for for installing them is almost nil as they are installed at the most economical time - at assembly.

If you can't afford it now - why not expand to an external SSD over firewire or ThunderBolt later. They are totally silent and small. It just seems the risks of DIY on a new mac make it better for most people to pay a trained tech to do the install.