Well, when I bought my 1979 DS II in 1996 a local dealer estimated the value between $1200 and $2400. The last time that I checked the NADA website the "book value" there was around $1800 +/-
Basically, if your boat is in great condition, the value is around that depending on equipment.
OOPS! I misread your question, I saw "Day Sailer" and thought you had a DAY SAILER. The O'DAY 19 will probably be in the range of $1800 to $3000+ depending on what is included (trailer, outboard, condition and number of sails, roller-furling for jib?, etc.) Not a perfect source, but there is a 1980 O'DAY 19 on eBay right now, watch the auction to see what she goes for.