Tough to throw out a price on the job, prob won't get any posts helping you there. That being said you can replace the run with PVC which will more likely be cheaper and easier. The joint to the existing cast will be made with a banded fernco. The "rubber coupler" you mentioned before but it has a metal band around the rubber.
You need only replace as much as is bad and can replace as much as is feasible. The cost to replace 20ft will not be double the cost to replace 10ft. Just like the cost to replace 20ft of the whole 40ft run will not be half. If it needs replacing better to do it all at once since it will be MUCH cheaper to have done in one shot.
Piping is usually in 10ft sections so the ease of getting materials in/out and working in the crawl space will most certainly factor into your bids. Getting cast iron into a crawl space with limited/difficult access will be cost prohibitive. Not knowing the layout it is difficult to know/recommend how much or little to replace. A main line is usually a 3" or 4" pipe, you didn't specify but we'll assume that's what you have. Don't just have one guy price it out for you, and make sure you have all contractors that bid specify the scope and materials of the work to be done. So you have an apples to apples comparison.