$400 to $450 is a reasonable price unless it has box, tools and papers, but these days a lot of folks are attempting to ask more (some a LOT more
) and some are apparently getting it.
Some will say the base price of a 95% or better 28 w/original grips has crept up to the $500 range. Not for many of us. In the last two years I have paid less and I have paid more, but for the "more" it was a 5 screw "S" numbered gun in prime condition. The "less" was over $150 less for a shooter with a worn finish.
Most any 28 in fair cosmetic condition and good mechanical condition should bring $400 these days. More than that, well, a lot of folks are convinced they will keep climbing in value. Buit the market overall is "soft" right now. I see people asking more, but I also see a lot of them sitting at those prices. YMMV.