45-90 USD
I THINK you mean a Carrero & Astelarra Vanguard. Single shot break open 12 g? These were inexpensive utility grade shotguns- value around $75-$90, depending on condition.
50-100 USD
H&R / Harrington & Richardson
1951 Model 37
yes it is a single barrell
Assuming you mean a top break shotgun, there were several models, prices will vary. For a single barreled top break, about $70-$75
Your Stevens model 9478 single barrel shotgun was made from 1987-1988.The value is between 60-85 dollars in the condition that you describe.
50-100 USD or so
No
This sounds like a "ZULU" shotgun. These were made from obsolete military muskets in the late 1800s, mostly in Belgium, for sale in various colonial areas.
35-100 USD or so
1901 single barrel shotgun, washingtion