Moralem and Watch Dog called it pretty well.

Ruger made 1.5 million of the Six series in total, but they've been out of production since 1988 (32 years and counting) and they won't resurrect them given the evolution of the design into the GP100.

They made just under 85,000 Bicentennial roll marked Six series revolvers, so they are not rare by any means. They are 5.6% of the total production so the odds are about 1 in 18 that a particular Six Series DA revolver you see in the shop will be a bicentennial model. They are interesting and it's nice to have a number of different models with the bicentennial roll mark, but not enough to warrant paying more than about $10 more for one.

-----

They also have an intrinsic value as shooters and if it is in very good or excellent condition, you can expect to pay at least $400 for a 4" or 6" Security Six and an excellent to near mint example can sell for $500-$550.

The 2 3/4" Security Six and Speed Six will sell for around $100 more, so expect to pay around $500 to $600 for one in very good to excellent condition.

The 150 series low back revolvers will bring a premium as there were only about 65,000 of them made from 1972-74. There's no firm cut off as Ruger pulled whatever frame was handy during the transition in no particular order, so you see both in the 1974 time frame.

The early 1972-73 fixed sight revolvers are almost always stamped "Security Six" rather than "Service Six" or "Police Service Six" and those will sell for a bit more as well.

----

For the revolver in question $550 is a fair price. $500 would be a deal, and $450 would be a steal of a deal.

That said, if you want one, $600 is a fair price as they don't show up very often anymore. People who have them tend to keep them. If you pass on this one, by the time you come across another one, the prices will have appreciated and you'll pay just as much or more.