Alternators cost about 60 to about 80 pounds new, Second hand ones cost about 30 or 40 pounds depends where you shop, As for removing them its pretty easy to do if you have some basic motor repairing knowledge, Best thing to do if you are going to fix it yourself is to take pictures using a digital camera from various angles before, So at least you can look at them on your pc to make sure you have refitted everthing right, Even better refere to the workshop manual
Ian Mechanic
October 2009