"hard" is impossible to answer... it really depends on your skills and resources.
expensive is easy... i spent about $180 on silicone, $20 on plaster, and $40 on misc stuff... wood to make the boxes, clay to plug up holes, disposable brushes, gloves. so a bit over $200 to make the molds.
it's definitely much harder to make the original than it is to make the molds... heck, even webchief was able to brush the silicone on 
there are TONS of tutorials, videos, guides, etc... online for making molds. there are links in the wiki here to some good ones.
it is expensive and messy, but fun.
as far as doing it in an apartment... there are no particular odors from it, so no worries there. but you'll want a good space to work where you can afford to spill some silicone. drop clothes are your friend.
