2000 Honda Civic Exhaust manifold and Cadillac converter

Hello! My mechanic says that I have a crack in my exhaust manifold, I know he's telling me the truth because he showed me the crack. I wanted to know if he's telling the truth about it being expensive to replace. He says he'll have to replace the whole exhaust manifold along with the cadillac converter because they're one unit. He says it'll cost about $800. Should I pay him the $800 or should I shop around? Should I buy the part myself (if its cheaper) and then pay him to install it? Please help! Thanks!