It might work. I have a home phoneline network, and I've found that when I plug a machine into the wrong phoneline it still works. But, it may be some quirk of my wiring, and the HomePNA docs I've seen always say you have to be using the same phone #. If I was starting my home network now, I might go wireless, but when I did it 6 months ago wireless seemed pretty expensive and I was afraid it wouldn't work well in my big house.
As far as cost goes, you can buy Homepna cards direct from Netgear for as little as $20 apiece. Combine that with using a PC as an always-on internet server, and you've got a pretty cheap setup. But, if you buy a router (which I found works much better) that can drive up your costs, e.g. the Netgear PE102 ethernet/HomePNA bridge (which you would connect to a router) costs $113 at buy.com, or the Netgear RP334 router/built-in HomePna bridge is $250. I got the latter and it works great, but in retrospect I can't say I went the most economical route. But, I am very happy with the performance of the network, which has 6 different PCs on it.