Say a stock good condition 4 bolt main bottom end with cheapie after market cast flat tops, .030 over, everything is balanced and uses a mellings oil pump and ARP hardware and some cheapie cast rods, a big flat tappet cam, rev kit, cloyes timing set, stock oil pan and timing cover with felpro gaskets should be about $500 if you assemble it yourself, you can find a good set of 487/x heads and deck them .030, stock size ferrera valves with a 3 angle valve job, comp cams beehive valve springs, titanium retainers and locks and a cheapie set of double roller rockers with ARP studs, should come out to about $600 you figure about another $500 for a new carb and manifold, aluminum short pump is about $150 and you can get a cheap set of used shorty headers for less then $150, exhaust tubing and an x-pipe is about another $130, I see rebuilt TH350's and 700R4's go on craigslist anywhere from $200-500, a higher stall torque convertor about $150, a driveshaft should be about $200 if you bring them the yokes, the mounts are $330, the 90* angle drive for the speedo cable is $70, the correct speedo gear is $10, a speedo shop will be able to make you a cable from the GM trans to the datsun speedo for about $50, a good condition nova 4 core radiator will be about $25, a taurus e-fan will be about $25, a reman alternator another $50-100, you figure another $50 in random nuts and bolts, $50 to make new radiator mounts, $40 for radiator hoses and clamps, $100-200 for a shifter and cable, and say $100 for misc gaskets, fuel line, fuel pump..etc
The prices are general, but I would rather have someone being prepared for a slightly larger budget, then having one too small.
You can also find ALOT of good, built circle track race motors with alot of street life left in them for under $1000 if you know where to look and who to ask.