Knoll Tulip Chair Cushions  

Seems like a good value @ 1/3 the price
I was wondering that, too.I am going to need one or two vinyl ones in the future.This is the guy in Texas,right?

They offer
A number of different colors... I was in contact with them about the colors, but they weren't very good about responding so I never purchased.

tulip cushions
We just finished making a set of 6 . They can be rather difficult to make, but you should be able to use the bottom foam rubber on the cushions as that is very expensive foam. but remember their is a inset piece. and you should use a heavy fabric like a maharm fabric. in Hallingdale. These are for the full cover.

I would bet a lot of money...
I would bet a lot of money that that ebay seller is using cheap poly-foam instead of latex foam in those cushions. You need latex for these---not only because the originals were latex but also because poly-foam doesn't have enough resilience. This is a 2" (?) thick foam on a hard surface and with poly-foam you will feel the hard surface under your rear. Latex foam has more cushioning power in this thickness.
I know this because I made some replacement cushions of poly-foam and they suck.
You can buy latex foam cushions from foamorder.com and trim them to the correct shape yourself but it's kind of a lot of work, and they still will cost $40-50 or so. They have a handy calculator on their site and I just guessed at the dimensions and came up with that price range. They also have different densities. I think firmer is probably better than softer for these chairs.
You can get some great Knoll fabrics on ebay for cheap and you won't need much in the way of yardage. So that's one positive!

Depends on the firmness
Poly foam is available in many firmness ratings.

For what it's worth....
I have a pair of yellow Bertoia side chairs and I bought a pair of yellow knockoff seat cushions from some guy on eBay. They were decently sewn, but the color wasn't right and they had little or no 'cush' to them.
When Knoll was having their annual sale, I bought two of the real yellow cushions from the late, lamented Retromodern.com and these Knoll cushions were 100% better than the cheap knockoffs!
I suspect that these Tulip cushions will be about the same disappointment.
I wonder if eamesbikini has tulip seat cushions. They do a pretty good job.

woodywood
It's not just firmness, though. Latex has a resiliency that poly-foam just can't match.
I've got some polyfoam that is so firm that it's like sitting on a brick, almost. It's still nothing like latex foam. I'm not even sure how to describe the difference but it's very easy to tell by feel.

Isn't most padded furniture...
Poly foam these days? I'm pretty sure my sofa is. The cushions are over 6" thick. I find it very firm yet supportive and comfortable for seating or laying (but not really for slouching).

Yes.
Most foam today is poly-foam, mostly because it costs less. Latex is really expensive. Poly-foam can be pretty good, especially in the short term, but latex is better and it lasts a lot longer.

Also...
...I say this as an amateur upholsterer who has tried really, really hard to convince herself that poly-foam is as good as latex foam, and who has admitted defeat.
I still regret not buying up a bunch of the 1" thick slabs of latex foam that I saw 3-4 years ago at PA Fabric Outlet for twenty dollars apiece. Grrr.