I've said this before- What's it worth to YOU? If you're willing to go $400, then do it!He//, if it's worth $1000 to you, then do it.
You'd be pretty hard pressed to build a respectable kart for less than $400. I've done it, and so have others- but you've got to be a wicked scavenger, very patient, and willing and capable to rework a design plan 100 times based on some oddball part you just got and didn't foresee. My kids' kart came in right around $200 including a new engine and clutch, but it sure wasn't easy. All steel was scrounged. Steering wheel from computer control. Front axle, spindles, arms, steering rack, front wheels, rear wheels and tires, differential, and rear hubs from an old lawn tractor- scored free. Brake master cylinder, hose and caliper from a motorcycle. Jackshaft and bearings from an industrial ventilation fan. Front tires from a garden cart. Pedals fabbed from 1/4" round stock, also scrounged. Two rod ends from old Honda ATV, the other two from unknown source. Scrounging for a low-buck build takes a lot of time and patience. Many of these parts literally took years to amass for as cheap or free as they were. Lots of luck involved, too. Not too often do you find a lawn tractor with a chain-driven differential axle and manage to talk your way into it for free.
In other words, just going out and buying everything you need that's going to fit and work out without some major logistics is going to cost you. $400? Get the unit you're looking at.