Sorry to say, most of the Quadraflex line had little value even when they were new.
Quadraflex was the house brand for Pacific Stereo - which originated in California before being purchased by CBS. During ownership by CBS, Pacific took on a bean counter mentality of making profit. To that end Quadraflex was also owned by CBS. The same applies to Transaudio (the entry level line) and Concept (the "high end" line) of components and speakers. There never was a Quadraflex company; no headquarters, no manufacturing facilities and no company (other than Pacific Stereo) to deal with customer service.
CBS was in the vanguard of off shoring electronics production. They hired a design team, made contracts with a subcontractor for building and for shipping the equipment and the end result was a CBS owned company sold its products to another CBS company - at a rather substantial profit even if no single Quadraflex component or speaker ever sold through a Pacific Stereo store.
Interestingly, NAD set up a similar business model in the mid-70's as a middle man only audio company who had no established manufacturing facilities and sent out quotes for low bid when a new design was due. Since then, more than a few audio companies have done more or less the same thing to greater or lesser success. Quadraflex, however, existed only while Pacific Stereo existed and when Pacific was no longer profitable to CBS, everything related to the venture was dissolved. All of the house brands ceased production and were never to exist again.
You don't mention which speakers you still own. The 15's and the 19's were the most popular models based on sales numbers. Both were acoustic suspension systems. Sealed enclosure systems have generally fallen out of favor in the modern market. I'm a bit surprised to hear your speakers are still functional as most of the sealed systems from that time have suffered from deteriorating surrounds on the woofers. However, to give you an idea of value; the ST19's had an MSRP, if memory serves me, of about $279-299 each. The store cost varied from day to day as CBS needed to show various numbers on their tally sheets, but generally was no more than $99-109 per speaker. That was the cost to the store which was inflated to provide a healthy profit to CBS as the manufacturer of the Quadraflex line. A more realistic store cost would have been in the range of $60-80 per speaker if we assume the manufacturer is allowed a roughly 40-50% markup. For comparison purposes a walnut Large Advent from the same time booked cost at about $79 and sold for roughly $119 per speaker. The Advent line was the best selling speaker in the US during the late '60's to mid '70's and was seldom discounted, unless purchased within a system which then typically received a 10% system discount from most Advent dealers.
The Quadraflex line existed to be discounted.
As a house brand, the purpose of the lines were - beyond making profit for CBS even before they were sold to a customer - giving the sales staff a healthy margin between store book cost and MSRP which they could either heavily discount or use to gain larger profit margins for system sales. At $109 per speaker cost to the store, the sales person could easily knock off half the retail price and still make a very reasonable profit while receiving a "spiff" for selling the CBS owned line. A spiff is essentially outlawed since it provides an unfair advantage to certain lines by handing out a reward to the sales person who sold the product. A $10-25 spiff on a pair of Quadraflex speakers could give a sales person a few extra hundred dollars per week in their pocket back in the mid 1970's when you could still buy a car for less than $3-4k.
Add to this the Quadraflex line was always positioned in the demo room to have the most favorable location for sound - in the corners and often on the floor or a high shelf which would add some boom to the speakers' output. Pacific did not use volume compensated speaker comparisons which gave the Quadraflex speakers a distinct advantage when selling against a higher quality line such as Advent or Acoustic Research.
All in all, it was a rigged game set up by CBS.
This is not to say the house brands were simply junk. Pacific also sold a "house brand" speaker made for them by Infinity and numerous phono cartridges with Pacific-only model numbers made for them by well known manufacturers. Most sales people felt the ST15's were a reasonably good speaker - and could be discounted to provide a nice, low cost system for the buyer on a tight budget. An all Quadraflex system could be put together for $299 and still make the store, the sales person and CBS a reasonable profit.
As to the actual value of your speakers; after selling audio for almost three decades, the one thing I know is anything is worth what someone is willing to pay. No more and no less. In this age of buying vintage this and that to regain your misspent youth, you just can't say what someone might want to give you for your speakers if they are in good condition. Another CBS owned company from the '70's would have been Fender guitars. A mid-70's Stratocaster is easily worth a few times its original selling price; more for certain versions. Of course, most musicians recognize the name "Stratocaster". Hardly anyone not involved in Pacific Stereo would remember the Quadraflex name.