Emma,
Depending on what you are using the traffic for, $200 *should* be enough to determine if buying more makes sense. It will give you a good idea of how productive (click-wise) the traffic is, and its breakdown geographically and by language.
But, if you want to gauge solely by paysite signups, it's not enough. You won't have enough data to make an informed decision.
We also can offer you traffic. Our (shitty!) info page is here;
http://www.fpctraffic.com/buy_traffic.html
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AdultAdBroker - Buy and Sell Your Flat Rate Banners, Links, Tabs, Pops, Email Clicks and Members' Area Traffic - updated August 2025