I'm not well up on networks but I think that for 2 PCs only you can connect them with a crossover cable without a router or hub or whatever it is which, providing cabling is practical, would be a cheap way to start/experiment.
100% right. I've done it and it works allowing 2 machines to share the same cable modem. Cost - I think for a 5m cable, I was charged about £5 by a small computer shop. Never seen one in PC World.