as far as completely building one for under $400... unless you already have all the components and steel, you won't be able to do it except maybe at used/salvage prices i'm guessing. at good new prices you're looking at $75+ for a valve, $150+ for a cylinder, and $125+ for hoses, elbows, and quick connects. you're at $350+ already, not including any steel. $400 is pretty much what it cost me to do mine last year and i did have all of the steel.
i think it all depends on what you want to do with it. if you're in a commercial business then you're justified in multiple thousands, but as a homeowner it's most likely a waste. i've split about 10 cord over the last two years, and i'm using a tiny little homemade splitter which does pretty much everything i will ever need.
this is a 24" oak log on my splitter. it was no trouble at all and that cylinder is only 3 1/2 diameter. the cylinder also only has a 18" stroke. for me, a 24" stroke would be next to useless. since i moved three years ago i have put two new wood burners in my house - a large regency f3100 stove in the basement and a medium insert in the fireplace. the basement stove can take up to 21" and the insert can take up to 18". i'm not going to cut two sizes, so i cut everything at 16" +/- so i can burn it in either stove.
because of this i went with the 18" cylinder and i built it so the gap between the plate and wedge is 19". i do most of my splitting by myself, so once i split, i let the cylinder retract while i toss the split pieces in the pile and get the next log. if i had a 24" splitter, i would often have the cylinder completely retract and then have to wait for that extra 6" of takeup each time i split -a colossal waste of time.
i'm also powering it with a 5.6 gpm pump, which will make many of the statisticians here cringe, but in my experience, it's not that bad at all. on the split stroke is may well be faster than most stand alone splitters. the reason is everyone looks at the no-load speed on the pumps. sure, you may have a 15gpm pump, but that's at low pressure and not under a load. when you hit the log, you're down to a 2-3gpm pump until you've popped the log in half and it kicks back into high gear. i'm running 5.6 gpm all the time. i'm also driving it with a 24hp engine, so there's no power issue. on the return stroke i do lose some speed to the two stage crowd since i'm handicapped at 5.6 gpm, but as i noted before i am generally a one man operation and i only have an 18" return stroke.
some people may not want to rack up hours on a $15k+ powerplant to run a splitter when a $200 engine will do, but for me (the average homeowner with a tractor) it's not an issue. the few hours dedicated to the splitting is minimal in the overall scheme. also, it's got a high stack so the fumes are overhead, and i run around 1500rpm so it's quieter than many of the cheaper 3600rpm small engines. i'm not saying it's perfect for everyone, but for me it sure was a no brainer. $400 plus labor to build it as opposed to around $1000-$1500 for a new stand alone. for me i also like having it be one less engine to maintain.
