exhaust manifold is cracked and what is a reasonable estimated repair cost

It isn't too compicated to fix yourself. Get a quality header, such as one from Banks, unbolt the bad one & bolt on the new one. Banks headers are pricey, around $500 or so, but totally worth it. But you can get other good headers for anywhere from $200-500. You'll probably need some anti-seize to get the rusted bolts out, and maybe some new bolts to replace the rusted ones, but just watch a YouTube video of someone else changing one out and take your time. I'm about to do it for the first time myself here pretty soon. May as well inspect your entire exhaust system too... Almost all of my support brackets on my muffler & tailpipe are rusted to the point of being obsolete, pretty sure each part is just rusted to the next one and that's what is holding it all together. Get yourself a decent cat-back exhaust, and if necessary a catalytic converter, and you'll be happy with your decision. You can do it all by yourself with quality parts for anywhere from $300-1000, just depends on what you're looking for. Or you can just get the stock exhaust manifold. It's just going to crack again, but you should be able to get 100-150K miles out of it. Not sure of the cost for these or what a mechanic would charge to install.