Value is too difficult to be accurate without seeing the gun. Condition which includes the amount of original finish and original configuration will determine value. Condition and age is also a factor.
Recommend taking it to a gun shop for appraisal.
You can also do your own investigation by looking at other models being sold that are in the same condition as yours. Try the gunbroker.com or a periodical called "The Gun List"
You can also reference the Blue Book of Gun Values which can be purchased on line at various retailers.
Topper are low end $50 - $150
They sell new for $125.00
$100 to $110 in excellent shape
Made through the late 1940's - mid 1950's. Value under $100.
50-150 USD
$100
100 USD give or take
About $70, depending on condition.
Less than $100.
About $100.
value of a harringtonand richardson 1900 28 gauge
Typically a used topper will run between $50 and $150 depending on condition.
$150 in mint condition.