well cost is totally going to depend on what you want the engine to do when you're done, what shape it is in now and lots of other factors. you could pay anywhere from 1500 to 9 or 10 grand building a motor.
also, you cant bore a 289 to 302, you need a different crank and rods.
porting and pollishing is a term that is way too loosely applied to engine work. you can port heads and intakes, and thats about it. i hear way to many people use "p&p" when they are just talking about basic head work, sorta like people saying that an engine has been "ballanced and blueprinted"
let us know what you want to do with the engine and how much power you want to make, then we can help you out.
Around here, a V8 is rebuilt for about $1500 with a .030 bore. Thats without port/polishing the heads and a "stock" rebuild. That includes putting it back in your car and using all of your old parts except pistons.