[Varyon]

GURPS typically overcharges for plate for gamist reasons (plate is game-mechanically better than mail, but historically had a lower pricetag), and I believe undercharges for cloth armor for the same. Also, the CF system doesn't really work that well for bronze - a relatively flat addition to cost based on weight would be more appropriate, and this "error" is particularly pronounced for the more expensive armor types, like plate. Finally, bronze was a type of material that had wild fluctuations in price (largely based on the price of tin), making the GURPS one-price-fits-all approach not work that well for it. The above "errors" were most likely chosen because they work better for gaming than, but when you put it all together, you do indeed end up with greaves as the most expensive armor pieces, by far, for a late hoplite.

Of course, the greaves and helmet are also the most substantial armor for the late hoplite, and considering greaves are heavier, it makes a good deal of sense for them to be the most expensive armor pieces. The rest of the body is largely protected by the shield (and those of neighbors, in a phalanx). The principe is in a similar boat, of course, although he has much more substantial torso armor (there, the price "error" between mail and plate, as well as the arguably-excessive multiplier for bronze plate, conspire to make the greaves more expensive).

As for greave design, they were clearly more of a plate than a segmented plate design, and looking at pictures online appear to cover the lower legs from the front as well as the back. The back of the knee isn't protected, but that's probably covered by Armor Gaps (although you could shave off a bit of weight and cost by leaving the back of the knee completely unprotected).