Many people starting out in business acquire funds from many sources, including savings or loans from friends, family and financial institutions.  Providers can't always count on support from these quarters.  

Despite the pleasant self-image we have, most Americans, providers included, are not "middle class" with tons of cash lying around.  I'm seeing a provider now who advertises in different places including CL [that's not where I saw her ad though], who does not charge premium prices, but she is a great provider and has gotten some great reviews.  People have to start somewhere.

I disagree with your assumption.
A person's seriousness about the hobby should not be linked to how much they spend on an ad.  There are some very good sites that do offer free advertising; should a provider be considered less serious because she is business savvy and takes advantage of the free exposure?
Someone could spend and spend and still not provide a quality service.  Pay attention to the reviews; they are usually a good indication of whether or not the provider is good at what she does.
Just because someone places a dime store ring in a blue box; doesn't make it Tiffany's.

There is certainly logic to your argument - that a provider's willingness to invest time and money into herself is an indicator of her likelihood of being legitimate. Unfortunately, despite the logic in your assumption, reality does not bear it out.

Eros is probably one of the most expensive ad sites, and in addition to being more expensive than most place, the hoops they ask a lady to jump through to get started are a bit more complicated than other sites. Despite that though, if you do a search for rip of reviews, you will find just as many "rip off" providers who were advertising on Eros as you will find from CL.

In essence, if you want a "sure thing" - an experience in which the odds of having a good time are weighted heavily in your favor - stick with well reviewed ladies. A long list of reviews are a far better indicator than anything else, since we don't get to control our reviews the same way we control the ads we run. Sure, a lady can fake a review, it happens despite the best efforts of TER, but she can't fake 20 reviews.

I check Eros most often, followed by TER, Citivibe & CL.
Are there other sites that should be part of my regular searches?  (Philly area)

Why spend money on ads when you can use it on lingerie and high heels? Which would you rather see, an expensive ad or some nice lingerie and stilettos? I put mine where it does the most good.

Many times "expensive" websites are merely egocentric at heart

I’ve noted in polls here and elsewhere the majority of men just want to see some good pics, prices, and contact info.

There is an incredibly successful reviewed provider I found on TER that uses a simple 1 page yahoo based advertisement. I know from researching yahoo's offerings that her page probably costs around $10/month. This provider is nowhere to be found on ANY escort mall in the region and to my knowledge she does not tour; so apparently her simplistic 1 page advertisement is her only promotion outside of her astonishing litany of reviews, and she is utilizing both masterfully.