Well, how much can I expect to pay for forged goodies to be put into a 78 350 v8.

I am looking to adding some nitrous.

Do the forged parts add alot of hp?

 

first off, forged parts add absolutely ZERO hp.
forging is an alternate way of manufacturing the parts that results in much greater strength.
How much the forged pieces will cost depends on what kind of forging it is (what alloy they use in the metal that they start with) and who makes it, etc...
When you say "some nitrous" how much yo you plan? 100-150hp and you should be able to run stock crank and rods, even stock/hyper pistons if you're careful not to run lean. I'd get forged pistons as good insurance if you're planning N2O from the start - $200-250 for TRW forged pistons, all the way up to $500-600 SRP or JE pistons.