Truth be told,$500 isn't that much... for the proper build.
That means clean solid workmanship, but as the standard assembly on a normal bike.
Custom fitting costs more, odd ball frames, mounts, shift kits all require more time on the part of the mechanic but that bike shown was obviously just thrown together.
I cant tell you how many people come into my shop with a "bargain" bike from a on line reseller and end up having to pay me extra to re do the bike, which we do happily, because we just scored a new customer and chances are if they are happy with the work and bike, they come back.
E caveat Emptor.
Check out any builders work before considering purchase.
I'm always happy to display our workmanship and answer detailed questions about any build from a rack mount friction drive 31 cc 4 stroke on a Schwinn beach bike to a full blast custom framed Morini.
Good luck
BBB