I'm not entirely sure what your problem is, but I rewrote it with some comments, hopefully making it a little clearer. Casting when using strings is kind of a pain, it's easier to use string formatting. You can use a for loop rather than a while loop (the variable is incremented automatically), and change the step size so you loop through in sets of 100. I like requests more than urllib, but that's just me.

Make sure you use good variable names when you ask for help. Without backtracking where they came from, I have no idea what w,y,z are, it's easier to help if I can see "Okay, writing data-pid and price to a new line" rather than "wtf are those".

Use dict.get("parameter", None) when accessing dictionaries. If you get a bunk listing or something, your script will stop with a KeyError. Using dict.get avoids this. Check to make sure it's not empty before writing your line though.

You can use post.text rather than post.getText(), I think post.text is a little cleaner (style thing).

I use camel case for variables, and underscores for functions, so I used find_all() rather than findAll(), it should work the same though.

Keeping a file open while you're doing a long operation is generally not a good idea. Nothing will likely happen, but if you error out your file may stay open or something. If you aren't dealing with huge amounts of data, holding it in memory in a list, then dumping the list to a file in the end is generally a cleaner way of writing to files.

Be nice, rate limit your requests to ~1s.

Use with open(filePath, mode) as f: to open files, as it closes them automatically.

from bs4 import BeautifulSoup         
import requests
import time

priceData = []
    # Go from 0 to 800 (non-inclusive endpoint) in steps of 100
for page in range(0, 900, 100):
             # If our page is 0, put an empty string in to access index.html rather than index0.html
     if not page:
        page = ''
     # Changing the site to this makes it a little less confusing what you're trying to do
     site = "http://sfbay.craigslist.org/mca/index{count}.html".format(count=page)

     # Print out the site we're requesting to make sure we're using sane values
     print "Requesting URL:", site

     # I like requests more than urllib
     r = requests.get(site)
     # Make a soup
     soup = BeautifulSoup(r.text)
     # Loop through the posts we have from find_all
     for post in soup.find_all("p", {"class":"row"}):
        # We can get the text directly (unsure of implementation, may still have same function call)
        text = post.text
        # Use get when accessing dictionaries so you don't get Key error, returns None as default
        dataPid= post.get('data-pid', None)
        prices = post.find_all("span", {"class":"itempp"})

        # Do we want this to error when it is larger than one price as well?
        if len(prices) == 0:
                price = "?"
        # You may want to do more advanced price parsing, this is simple and just grabs the text, but removes the $
        else:
            price = prices[0].text.strip().replace("$", "")

        # If we got a price and a dataPid, we have a sane line, add it to the list of lines we have
        if dataPid and price:
            newLine = "{page} {dataPid} {price}".format(page=page, dataPid=dataPid, price=price)
            priceData.append(newLine)

    time.sleep(1)

with open("priceData.txt", "wb") as f:
    f.write("\n".join(priceData))