How much it costs completely depends on how it's built. A reliable street v8 that makes good power like a 302-347, built to take abuse and with good parts, is going to run upwards of $10,000 for parts and labor. Hell, these days good heads alone are anywhere from $1,200 and up, good pistons can easily run over $500. Then the labor to actually check everything the way you're supposed to, better connecting hardware, good gaskets etc. A reliable high output engine is not cheap. $4,700 is pretty cheap for a 347, but that's because it's using knock-off brand heads etc.