To the question of whether or not its cheaper to build than buy, I say if you are a resourceful person, you can certainly build cheaper than you can buy. A few trips to the local scrap yard recently has yeilded me a 4 1/2 inch boreX 28inx 2inch cylinder for $20bucks. ($.16 a lb). They have a prentice knuckleboom loader they are scrapping that will produce 6 or 7 other rather large cylinders for $.16 a lb. Need Hbeam, $.16lb. Flat plate, $.16lb, a verygood 300 6cyl, (I'm building a processor) again $.16lb. My point, There is no rule that says you have to buy new parts to build your splitter, the scrap yards are full of very usable parts for scrap prices. If you dont care to get your hands a little dirty, you can build a splitter pretty darn cheap, including any and all options you can think of.
The 42ton splitter at Northern is $6400something and weighs 2400 or so lbs. at $0.16lb thats $384. I can invest a lot of time to save $6000.
The real question shouldnt be, can You build cheaper than you can buy?, Instead, Why buy when you can rent for $50 a day. Now if you are just wanting to split firewood for your own use, renting can be a very viable option. My local rental places keep pretty new and reliable splitters, rent it, use it and take it back, no need to worry about storage or the durn thing always being in the way taking up space.