I can't help with specifics regarding the 105 components, but I purchased my 1990 SR300 in October 1990 for $320. This was the least expensive 3.0 series at the time and was originally equipped with Suntour Blaze components. Yes, I am still riding it, though the components have been changed.
I had wanted 105 components at the time on advice from my brother, but remember that the price was significantly higher, though I don't recall how much. At the time, my previous bike was a well used Schwinn Varsity so I was more than willing to settle for the Blaze components. The components were upgraded to a combination of Campy Chorus and Athena around 1994 and these are still in use today.