Old Home
I am considering buying a 60 year old home that has slight sagging in roof due to undersized rafters. And theres a slight sloping in the floors due to weakened floor joists. How serious are these problems? And how much would it cost to have fixed? 
how did you know that the roof is sagginf due to undersized rafters? did the owner tell you? this needs to be looked at closer. get a contractor to go up there and look at it for you. it could be a cheapy thing but most likely not. the sagging could be undersized rafters but it could also be that they're weakened from water rotting if you have holes up there. it could also be just that they used slightly less than 3/4" wood to cover the joists. just check it out.
also, same for the floor joists. a leak in the roof could've let water make it's way down to the floors and soaked everything up. levelling floors isn't the easiest thing but it can be done if it's only in the kitchen, say. if it's every room downstairs then you're in for some work.
i don't know about where you are, obviously, but if i replaced the roofing (no such thing as fixing, truthfully) and levelled the floor, i'd be looking at a few hundred in wood but about 3 or 4k in labor and 2 to 3 weeks. so much can happen with this.
there could be 5 layers on the roof which takes more time (read: money) to fix and who knows with the floors. i just took up a floor in the kitchen and bumped into 8 layers of tile/linoleum/plywood/paneling AND nails. this is the dirtiest work man ... whew!
I agree with Jackman,
You need to have a professional look at it. Simply jacking up a floor or roof without knowing the cause can be a problem.
If you use a contractor then make sure they know basic load bearing principles.
I have seen all sorts of quick remedies..........raising roofs and floors properly mean that approved footers be done in order to carry the loads.
Also any wood members that are underated should be corrected and additional weights removed.
I have gotten in long discussions about this very topic and posted articles on this site about such occurrences.
Bottom line......seek a professional to evaluate it.
NC