Yep, the C-orgs like WS shuffle their line up rapidly. You have to be willing to a) gamble yourself b) when credible positive reviews do roll in, you have to act fast, like within a few days.
As an example, WS recently had two decent Colombian girls in their 20's (one was fair skinned) at the $140/hhr mark. Basic service, but close to their pictures and a bargain at that price point. Mongers posted several credible reviews on 141. But as of yesterday when I texted the PO to repeat, they are already gone. Poof.
If you really want to have a higher end white girl experience, Privatedelights is probably going to be your best bet. But like Sexyloser said above, take whatever you might spend at a C-org or K-org and double or triple it. It's also much harder to find reliable reviews on white girls. PD reviews serve a different purpose, it's all about getting a referral from a girl, so you just get pages and pages of useless glowing reviews that don't tell you anything. The negative reviews get rejected from the site and deleted.