I found out a bit of info... I can't for the life of me remember what site I found it on, but I did jot down some notes while reading...
There were a couple different patterns of Fraggle Rock PJs made, but none of them were manufactured for very long as they didn't actually sell all that well here in the U.S.
Today, they are rather rare to come by as most who owned them actually used them to the point that they were worn out and thrown away. Those who didn't wear them out passed them down in the family and as such, teh PJs were worn out by a younger sibling. If you can even find them these days, they most likely won't be in very good condition, so if you found a set that seem to be almost unworn, I'd say you have a bit of a Fraggley treasure.
Unfortunately, they aren't very highly regarded by most Fraggle Collectors nowadays. But, those that do seek them out are usually willing to pay a relatively substantial amount because of the scareity.
Personally, if it were me selling them (which if I had them, I probably would never do, but just for example...) I'd start an eBay auction off around teh $20 mark. You might not get that, but if the right buyer or two comes along, they could skyrocket in value.
So all in all, it's really hard to put a value on such an item as it could go either way depended on who happens to stumble upon them.