Moby Dick wrote:
Thank you for the informative reply. You mention fiberglass versus
rotomolded plastic. However, I see many more materials on various web
pages:
Carbon
Kevlar
Fiberglass (as you mentioned)

These three are all forms of composite, with the major difference being
that carbon, Kevlar and carbon fiber/Kevlar boats are typically lighter
than the same model in fiberglass. They have the potential to be
stronger and/or stiffer, but most manufacturers use these materials to
cut weight instead.

Proprietary Lay-up (whatever that is)

It could be pretty much anything, but most likely it's marketing BS.
When you get right down to it, ALL layups (the number, shape, location,
material and order of the of the plies) are proprietary, since every
boat model is different, even from the same manufacturer. Some
manufactures incorporate polyester (Dynel, Diolene) or Nylon into their
layups for specific purposes, but it's nothing to get excited about.

Fabric-Skin frame (I can imagine)
Hypalon (whatever that is)

Skin-on-frame boats come in two basic types, commercial folding boats
and home-built (folding or non-folding). The frames are typically wood,
or aluminum tubing with wood or high UHMW polyethylene frames. The skins
on commercial boats are fabric that's pre-coated with Hypalon or
Neoprene, which is cut into panels that are glued together to form the
skin. Home built boats commonly use polyester or Nylon fabric sewn
around the frame and coated with polyurethane varnish, though neoprene
and Hypalon can be brushed on if desired. Pre-coated fabrics can be
used, but they're more expensive and harder to work with.

Functionally, the biggest differences between all these boats are
weight, cost and portability.

Commercial folding boats are the most expensive - commonly costing over
$4000 - and the most portable. Most tend to be on the short side (under
16') and rather wide, which limits their performance. Weight-wise,
they're in the same range as carbon fiber and Kevlar touring boats.

Boats built with carbon fiber and Kevlar are the next most expensive
(~$3000 - $3500) and are available in a broad range of designs. They
typically weigh between 5 and 15 pounds less than the same boat in
fiberglass and cost between $500 and $1000 more.

Fiberglass boats are the mainstay of the mid-upper end of the market.
They typically range from $1500-$2800. They're good workhorse boats that
can take a lot of abuse.

If you want to build a boat, you can make a skin-on-frame boat for under
$200. Stitch-&-glue and cedar strip boats can be built for under $500.

Is there a comparison of all these somewhere? TIA

Not that I've seen, but you might want to look on Kayak Wiki:

http://kayakwiki.org/index.php/Main_Page