Depending on condition and exact model, between $100 and $200.
1971
"P" prefix indicates 1953
50-100 USD or so
Your harrington and richardson model 900 was made between the years 1962-1973.
25-75 USD or so
10-130 usd
60-100 yrs old
No such maker. However, there is a firm called Harrington and Richardson and they did make pistols in 38.
50-100 USD
10-100 USD or so
Blue Book of Gun Values will help.
Trademark of Harrington & Richardson Arms company