Cheaper tyres tend to have wire beads, more expensive ones are folders, there are some - e.g. Vittoria Rubino and Rubino Pro - which come in both versions
- as JtheM says, the folders are 100g or so lighter.
100g, 200g for both wheels, isn't a lot if you have a heavyweight bike but if you're a weight-weenie and every gramme counts...
Although if you are to save 200g anywhere on your bike, perhaps the best place to save it is on the wheelrim, as it's weight (and hence inertia) you don't have to overcome when accellerating up to speed. Saving it on the wheel hub would have nothing like the same effect.
I wouldn't agree that folding tyres have more TPI, threads per inch - at any rate not
because they're folders.
Higher quality tyres have higher TPI
and have a kevlar folding bead to make them lighter - but it's being higher quality (and more expensive) that means they're
both high TPI and lighter/folding.
Cheaper tyres are made from cheaper materials, lower tech, less TPI, are heaviest, are wire-beaded.
I have fancy folding tyres on my lightweight 'best bike' but wire-beaded ones on my winter/wet-weather/part-time commuter bike - it's got mudguards and a rack so 200g makes not a lot of difference.
Apart from weight, unless you're touring and want to carry a spare tyre, whether the tyres fitted on your wheels fold or not makes not a lot of practical difference.
Folding tyres aren't easier to fit if you p*nct*re (some folding and non-folding tyres are easy to fit, other folding and non-folding tyres are sods to fit...)
I don't know what the raw materials costs of a kevlar bead are compared to a wire bead, or if there's any difference in manufacturing costs, but e.g.
Ribble are selling the wire-bead Rubino at £7.65 and the kevlar-bead folding Rubino Pro at £13.75 : I can't believe there's £6.10 difference in the cost of the tyre, so there's got to be some 'what price can I sell it at ?' going on here.
If everything else were identical on two tyres, you could argue that folding tyres should actually be cheaper because there's less storage/warehousing and transport/postage charges on something which folds down into a small package rather than a heavier tyre-shaped and sized thing !