it can be, but not always. i used to share a four bedroom place with one other person for $900 a month in fishtown. this is with a basement and big backyard (stay away from port richmond, though; **** is boring, and gangs of bad little kids do **** like throw rocks at girls or try to pick fights with grown men)
craigslist isn't your best bet, by the way (unless you're subletting). word of mouth is the best way to find good rentals at good prices.
also, find places or neighborhoods you like, then spend a day hanging out there, visit stores, check out stores, get a feel for what the neighborhood is actually like; drive bys don't work, and you might fail to see that the place with the gorgeous kitchen is three miles from a grocery store or has no restaurants nearby.
find a local coffee shop* and ask the barista about the area. often times, you'll get a slip of paper with an awesome landlord's number.
you'll also get a good feel for who lives in the area. what one person considers a "bad part of town" can often just be non-whites in an affordable area. Look at crime stats to get a real picture of the city (by zip code):
http://philly.everyblock.com/crime/
for example, compare east north philadelphia:
http://philly.everyblock.com/locations/zipcodes/19122/
to center city (in an upscale neighborhood):
http://philly.everyblock.com/locations/zipcodes/19107/
The upscale neighborhood has substantially more crime reported, but you'll still hear people who don't know the city complain that east of front/north of gerard is particularly dangerous (it's not).
*i seriously recommend moving only where there's a cool locally owned coffee shop within a five minute walk from your house (starbucks or bonte doesn't count). i've found that the best places to live are near locally owned coffee shops.
also find a good place for brunch (someplace that serves drinks!). once you find these two things, everything else will fall into perspective.