expect to pay somewhere around $900-1300 depending upon vendor, current deals, etc. for those. not bad, but i still would buy them from whoever has the lowest price at the time and not rely on ZZP to be your go-to source.
the way the spring rates work is like this: if you want to be really low you need stiffer spring rates to keep the wheels and tires out of the fenders. a softer rate will allow too much compression of the spring and cause conflict between these pieces. most of the springs that are recommended, or what they come with, will be some compromise between ride quality and handling. if you keep your wheel and tire specs in check you can probably roll with these without issue and still get the car ~3" lower than stock, maybe a little more. beyond that you're going to want stiffer spring rates and to fine tune the front versus the back to aid in handling.
typically the stiff springs you have the more flat it will handle. the stiffer springs you have in the front the more the car will want to understeer. the stiffer springs you have in the rear the more the car will want to oversteer.
the suspension style on this car will dictate that the front spring rate is higher than the rear for more neutral handling by about 15-20%. it'll also make sure the ride quality isn't as harsh. the rear of the car being stiffer will make more different for harsh/sharp bumps/dips in the road than the front will. the front also has more weight over it.
too stiff of springs and the car will want to pogo off of bumps and the tires will leave the ground. this is bad. too soft of springs and the car will be floaty and bathtub around corners. soft springs can be partially counteracted by larger sway bars. too stiff of springs cannot.
because i know that the spring rates from BC are going to be measuring in kg/mm i would, off the top of my head, recommend a 8k/6k (448lb/336lb) front/rear setup for the average user. increasing spring rate by 2k (front and rear) for people wanting to go lower than the norm. however, i'd be interested to see what BC/ZZP figure out as appropriate rates for the wbody.
is that enough to be dangerous? heh.