How much will the traffic cost?

Prices vary a lot, and depend on which kind of traffic you buy, for which OS, and which countries you want to target, so that's difficult to say in advance.

Here's a list of the minimum prices:
https://www.plugrush.com/advertisers/prices

We recommend you set up a test campaign to try out. When you choose the settings you want for your campaign you will be able to see how your campaign rates against other competing campaigns.