I am looking at options for a 500 horse big block chevy build. I believe buying an already built motor from the classifieds here is my cheapest option. But also could be the riskiest. Cant be 100% sure of the quality/ condition of the motor. The other option I am looking at is buying a motor from a reputable builder like Tyler Crockett. I am curious to what a motor like this would cost me ?

Why don't you call Eddie Young, Tyler Crockett, and Teague and find out?

Why do you think Merc motors cost so much? Its not because they are faster or last longer, its because they are a known commodity and you know what your are getting (usually).

Piece of mind costs money

What I did. You can compare if you are thinking of building your own.

1987, MIE/340 (7.4) 454 4 bolt main Big Block
Dip it and hone the decks and cylinders cut .060 over
Block was detailed with all casting debris removed
Scat 9000 Crank 4.25, 496 stroker
Scat 26385P - 6.385 in. Connecting Rods - Bushed -I Beam Forged 4340 steel
Howards Cams 120255 - Max Torque Retrofit Hydraulic Roller Camshafts, Hydraulic Roller Tappet, Advertised Duration 288/294, Lift .585/.610, 112 L/S
Howards Matching Valve Springs 98636 - 130# / 330#
Cloyes Gear 9-203 - Cloyes Wear Plate, 031 in. Thick,
Maxspeeding Hydraulic Retrofit Roller Lifters
Probe 496 Dome Top +18.0cc - SRS Piston Set
Clevite 77 Cam Bearings
Federal-Mogal 4400 M20 Main Bearings
King Engine Rod Bearings CR849HP
Cloyes true double roller timing chain
Mahle - Plasma Moly Coated piston rings
Melling 10778C (Anti-Cavitation) oil pump
Melling HV oil pump shaft.
New Harmonic Balancer
Rotating Assembly Fully Balanced
Comp Cams High-Tech pushrods #7954
Completely refreshed Chevy Heads 14096188 / 454 Open / 118cc / 3angle valve
APR head bolt sets
ARP 1.90 rocker studs
Norris S/S 1.7 roller rockers
Seal Power Gasket set
EDELBROCK BBC R2 AIRGAP INTAKE
Pro Comp 2752 Fuel Pump
Pro Comp Ignition
Pro Comp distributor
Super Stock coil
8mm Accel Hi performance wires
New single wire alternator
Mini (gear reduction and fully marine) starter
Edelbrock 800 carburetor
12" aluminum, oval air cleaner (with K&N filter)
Brass freeze plugs
Stainless steel bolt set
4” Stainless steel risers
496 MerCruiser exhaust manifolds with Tall Risers
Sherwood motor mounted raw water pump

I would say you are around 10k to have one of the mentioned builders put you one together from start to finish. Last I spoke with Eddie he had some spare 502 blocks around. Please take a lesson from the many people that went down the used road and bought motors based on what the seller told them.DONT! or BE VERY CAREFUL! Although you may pay more up front, you will get a marine motor from these guys. If it is not built with the best attention to detail and clearences it will work but not for long.
I bought 2 blocks and started from scratch, callies, isky, manley, JE, Blowers, port polish blah blah blah and 20k later you have 2X650 dependable HP for years to come. I do know a reputable guy in Duluth if you need a recomendation.
Best of luck!