An hour's work is a pretty good quote considering how deep into the iMac that factory memory is.
Consider this, to service your iMac in order to access the factory memory slot you'll need: a custom service stand, new thermal paste, and various Torx drivers. It's not for the faint at heart, nor the non Apple Desktop Certified.
Ask the Authorized Service provider if they'll allow you to provide your own supplied RAM (obviously purchased without their markup), and have them install it for you. If you max out the RAM you'll never have to worry about it again.
If you start hacking at your iMac, be advised that if you break the thermal seal you MUST properly re-apply thermal paste to the mating surfaces. Failure to do so will result in potentially catostrophic overheating.
My advice is to go the Authorized Service Provider route and max your RAM. Shop around, not all Service Providers gouge...
Good luck!
