its about a 3 hour job and the cam chain from euro car parts in £80 mid range highest was 115
so average garage costs are £35 an hour so. 35x3+parts = £225 so i would expect it to cost between that and 300
Best advise in this thread so far ^^ depending on labour i wouldn't expect to part with more than £400 for this.
Iim pretty sure corsas have a reputation for noisy chains as it is. They have a pretty long life span to be fair so it probably isnt as close to breaking as it might seam. Either way i would have it replaced ASAP.
I doubt that the 20 year old was intentionally tying to pass off a car with a known issue, its probably been making that noise for some time.