Shark-
Are you considering a career or a hobby? If you're thinking about going all the way, you're probably better off borrowing the whole lot (about $45,000) and getting it all done as fast as possible. The tendency with getting all your ratings by paying a class at a time is that the money and time never seems to be there after a while- there always seems to be something else that needs to be paid or something else that needs your time and attention.
If it's just a hobby, cool. I did my PPL a class at a time, just as a hobby at first, like you want to do. I paid $67 for a 172 (demand for gas was lower then) and $25 for an instructor (so was demand for CFI's). After books, charts, whizwheels, plotters, examiner's fee and a heasdet I was out about $6200, and it took me six months worth of spare time. Mind you, once I was hooked I went hog wild, borrowed out the wazoo and now I'm flying for a living, in debt up to my eyeballs, scrimping and saving just to eat and keep the lights turned on, and loving every minute of it.
To answer your question more accurately let's assume a typical instructor rate of $35 an hour, and an average of 65 hours total time:
($75 + $35)X45 hours of Dual Recieved=$4950
Plus:
$75 X 20 hours of Required Solo=$1500
Plus:
$35 X 15 hours of ground school=$525
Plus:
PPL Gleim's (excellent study guide for the written)- $16
PPL Written- $60
PPL Practical Test Standards- $6
PPL Oral Exam Guide- $6
Plotter- $10
Charts- $16 (2 @ $8 apiece)
E6B (whizwheel)- $12
Fuel Sampler Cup- $6
Headset- $175
Examiner's Fee- $300
FAA medical-$60
Comes to a grand total of $7642.
This is at best an educated estimate, and may go as much as $500 either way depending on the school, the region, and what bits you add or leave out (like the headset, sampler cup, and other important-but-not-absolutely-necessary items), so jrh's estimate of $7000 to $7500 is pretty accurate. At the very least, you have some better idea of the miscellaneous stuff that goes along with it.
Monthly cost? At a minnimum of 2 lessons a week (trust me, don't go lower than this or you wind up spending half of each lesson reviewing stuff you learned last time- very expensive!) at 1.5 hours per lesson gives 3 hours a week, 12 hours a month. That's 5.4 months to get 65 hours, so lets call it 6 months after cancellations, weather, and maintennance. $7242/6= $1273.66 a month in flying lessons, so you'll spend about $320 a week. The point? SAVE UP BEFORE YOU START!!! DON'T GET HALFWAY AND QUIT CAUSE YOU'RE OUTTA CASH!! SAVE SAVE SAVE SAVE SAVE!!!!!!!!!!!!! Needless to say, I've seen guys do this a lot, and some were really good pilots. The best advice I got when I was getting my PPL was "Just don't quit". It happens all the time.
Mind you, if flying is to remain a hobby after you get your PPL, it's a good idea to sock some money away each month ($300 or so,) to pay for it, and put it somewhere that you can't get at it easily. Hide it in a sock, put it in a savings account, whatever, but have it available each month for flying. Lack of funds is one of the big reasons people get behind on thier currency, and that can be kinda dangerous. Moreover, there's always something new and exciting in flying that you'll want to tackle (IFR, glider, aerobatics, tailwheel, etc...) and you'll want to have the money around to be able to do it.
Hope all this helps, good luck and have fun!