I have watched Wilkins and Wontkins bring all kinds of prices on eBay over the years. I have two sets, and I'd say the average I've seen is $300 per set. I've seen them sell for under $100 each or over $500 as a set. There's no real trend over time; sometimes they're up, and sometimes they're down. Despite their age and material, most I've seen are in very good shape.
Even more unpredictable are the Spanish Vicma muppets. Boxed ones go anywhere from single digits to several hundred dollars, though I'd say they have dramatically trended downward over the years (unfortunately for me, as I invested in several duplicates when they looked very rare and hot).
Another fluctuating value is the Herry Monster puppet, which was my "Grail" for many, many years before they started showing up on eBay, hence my user name. The most recent was a boxed version in beautiful shape except for some light scratches on the pupils. It went for over $600. I've picked up a lot of them on eBay, from under $50 to over $200, depending on timing and condition.
I have wanted to pick up the Ideal Rowlf and Kermit along the way, but don't love them enough to bid what they usually go for.