Below are two pictures of an all original 66 no-dash in the 2k serial range. I am the original owner of this pictured 66. It remains NIB with the original sealed tools, box and included literature of that era.
Values of these guns can range widely. Many were modified as has been mentioned, particularly in the sighting area, and like all collectible S&W handguns, those modifications can cause a significant impact on what the firearms current value may be, especially to serious collectors.
People added black rear sights, some had a good gunsmith add a red ramp to the front blade, some installed a target hammer and trigger, all modifications done to personalize, and in their opinions, to improve the look and performance of their gun, not caring or thinking that someday those changes may have a significant impact on the value of their guns...
Today we often see an anxious desire to take these modified guns back to their original configuration as many of us have seen the value of original guns skyrocket in recent years, and there seems to be no end in sight.
So what is a 66 no-dash worth?
For shooters as well as collectors condition should be paramount. For collectors as well as investment seekers the selection process becomes much more critical. I look at what may be missing and proceed from there. If it has an after market red-ramp installed restoring the gun visually to original condition is highly unlikely. If it doesn't have all stainless rear sights I have seen them selling recently for $125 and they are becoming increasingly difficult to find. The same for the trigger and hammer. If they aren't original you can expect to pay a bunch for them. If they don't have the original "football" grips you can expect to put out at least $60 for an average pair.
Then there is the issue of the gun having the original box, tools and literature from that period. I've seen the boxes for that model sell for $50 and up and those didn't have an original label with the serial number matching the gun. I've seen sealed tool packs for the 4" k frames, including the screw driver, swab, brush and rod sell for $35 and up. Another issue to many people is the lowness of the serial number. The earlier the better for collector interest.
When someone asks a value on such a sought after gun like the 66 no-dash the range can often be shocking. I've seen them sell for as low as $350 all the way up to $1375 in recent years. Just determining the model to be a 66 no-dash doesn't automatically mean it is a top dollar example.
Just owning a 66 no-dash is truly like owning a piece of handgun history. They are only the second stainless handgun and the first stainless K frame produced by S&W, the first being the Mod 60.