push mower is a no way man thing for me.

i did find you can get threw spindles for about $60. the steel to make a decent deck could get high $, i have a few 3'X6'X 3/32" plates that was my intention to use. so i never checked into that part. buying one looks to be a $999 and up option. it looks like a fairly simple design. could you build one for less $$ unless you had some parts on hand i tend to think it would be close enough that you may as well just buy.
starting needs would be
motor $400 ish for a decent sized vertical shaft
spindles $180 for 3 spindle option
pulleys run about $15-20 each, you will need at least 5 of these but 6 would be a better design.
need a belt $30 and up
blades $20-30 each (need 3 )
wheels, the easy option would be 2 each $10 HF 10" tires
steel to build ??? need thick plate at least enough to mount motor and spindles. also need some pipe and or bar steel to make adjustable wheels and a pull bar of some form.
then there is all the lil stuff that adds up we forget to include in the build price like springs, bolts, nuts, washers, grease fittings, grease, and definitely some good paint would be in order. and probably some other stuff I'm not thinking of at the moment.

all of the above should get you decent 46" or so cut pull behind depending on the motor size and number of spindles you chose to go with. all that can easily run over $1000.00

the only reason i was looking into building one was the fact i had most of the stuff on hand ready to use.
such as, i had the fallowing.
motor, spindles, pulleys, blades, wheels, and even all the needed steel to do the build. so it was really just about needing a idea as to how to put it all together for me. so my only problem was working out the how too part. a close look at one at TSC will show the simplicities of these things.

you just need to buy every thing in mass to keep the $$ down, ( would probably help to have it assembled in an out of the US location )
but to build one from scratch. if you are just looking for a project and want one. then you could easily end up with a much nicer than the one you could buy if you are willing to put the time and $$ into it. i you have the time and $$ it would be a cool project.

for me i have moved onto a fully functioning rider, so the build has left the building. along with the extra motor.