"Well, how much you got?" asks the unscrupulous flight school owner.
Its been covered here already but many variables here.
I did mine in 6 days for $1600, but it was a school that specializes in the program but is no longer in business. It is mostly preparation and studying, leading up to the training.
The flight portion should be easy. If you can pass a commercial checkride from the right seat, you should be good to go. If you haven't flown in a while or from the right seat it may take a bit of money to get current and comfortable.
Study FOI, CFI, and FAR/AIM. If you are on a budget you don't want to have to pay someone to teach you what you need to know. Find a CFI or school that has done a CFI training and knows who you will be taking the test with. I feel this is important as anything else. Getting a good gouge on the checkride can save you plenty of time and money and possibly prevent a busted checkride which is not uncommon on CFI initials.