$5285 for a 461 stroker with forged pistons, rods and crank seems reasonable, considering the rotating assembly often costs $1650-2100 balanced (depends on the brand of pistons and rods used too- be sure to ask what you are getting). $600 of that final cost is core charge and shipping,so $4500-4600 for a 461 long block is a typical price. I would also like to know what was exactly machined- a quality build should have been bored/honed with a torque plate, zero decked, main saddle align honed, stainless valves, heads serviced with a multi-angle valve job, hardened exhaust seats, bronze guides installed, guides machined for PC seals, intake surfaces milled to match the amount of material milled off of the block side of the heads.
I would also be concerned about what number iron heads were used- there are numerous Pontiac heads that have a 96-102cc combustion chamber. This size chamber will yield a 9-9.5:1 SCR on a 461, allowing use of 91-93 octane. The builder should also tell you what size pistons were used and how much the block was overbored- I don't think you'd want to buy a 400 block that was bored .060, which does yield 468 cid, but doesn't let you reuse or rehone the block should it need servicing. It's becoming harder and harder these days to find a standard bore 400 core, so I would be surprised if your builder started with a .030 over block- be sure to ask before putting the deposit on any build.
The "extras" listed are for the most part essential in a high performance build- you don't want a new cam installed with old valve springs, or stock used rocker arms. Full roller rockers aren't absolutely required, but do reduce heat and friction generated- at the very least you should run roller tip rocker arms (not as noisy as full roller rockers but better than stock rockers). The builder can also get a stroker kit with a cast 4.25" stroker crank that will save you around $300 as well- since you are shooting for 400-450 hp/ 500+ ft-lbs a forged crank is overkill and not required. No mention of 4-bolt main caps were made- not a must, since Pontiac 2-bolt main bolts can handle 500+ hp, but you should ask your builder about installing ARP main studs before machining the block.