Perhaps she realized she was undercharging for her market.  But perhaps - just a thought, yet near assuredly - perhaps as an agency gal she has no say as to her own rates.  

Is it justified?  Sure.
Do you have to book time with her and pay it?  No.

...a provider is worth exactly how much she can get. So if the lady can now get 500 when last week it was 300, then good for her. Perhaps if she does not get the volume of business that she wants then the new rate wont stick and she'll be back down to 300.

Locally to me it is not unusual to see providers listed under numerous agencies with differing rates at the same time and I recall one internet lady from the late 90's. Simultaneously she was £150ph on her indie site, £300 on one internet agency and £1000 through another high priced agency. You gotta pity the guys shelling out £1000ph for something apparently exclusive when Joe Public was getting the same thing for £150.

If the new agency is reasonable then make them an offer. You may be able to negotiate.

She's 'justified'in charging whatever rate she chooses.  You're justified in not booking her if you think it's too high.

What kind of cut are they getting?  

If she's new to them, they might adjust her price downward later on.  The higher price might not turn out to be as profitable.

Best solution is to find another provider if you don't agree with the price.

ALL pussy has a price and none of it is worth the price. That being said, seek out trolls who you can comfortably pay for but if she's working for an agency, she's likely paying 50% or more back to the house and if I was her, no pimp is worth 50% of anything, but spit on his boots.

I saw an independent provider a couple of weeks ago that was $300 and has now up her rate to $600 with a couple of hour minimum.  Granted she was new to the business and was a bargain for $300 but... Apparently some of her clients told her she was charging too little and helped her to set her rates.