If your paying to have it done then the cost of the job is going to be approx the cars value. Personally I'd run it till it ran no more unless you can do the job yourself. When they fail they generally jump a few teeth on start up on the exhaust cam leading to non start and providing you have not cranked the hell out of it theres a good chance no damage will occur. Again the cost of repair will exceed the cost of the car.
As Marks said, the cost of paying a garage to do the timing chain will be close to what the car is worth if not more.
They should be done roughly around 100,000 miles or 10 years which ever comes sooner. How ever most will never make this before needing doing due to having a poor service life from the start.
Replacement is quite a simple job with basic tools and the locking kit but time consuming as lots has to be removed to do it.
Cost of parts will vairi from £100 to £200+ depending on the quality of the bits you use. Some after market kits will only last a few 1000 miles before they need doing again. It really is a case of you get what you pay for! While you are doing it its also worth replacing the water pump and thermostat as they have to be removed anyway.
What oil are you using? if 5w30 switch to 10w40 and change it more regularly (5000 miles/once a year) it may help keep it quieter.