You have access to the pool of participants on all of our plans (Free, Growth, Enterprise).
Recruiting via the pool of participants costs the same, no matter the plan you are enrolled in. (we have discounts applied depending on the amount of purchased credits)
The cost of the study depends only on the total study duration (provided by you). Thus, you can select as many attributes as you like and you can make your Prototype as complex as you prefer without extra cost.
You can find more information on what happens when study's duration is over or under estimated here.
After creating a new "target audience" for the Pool of Participants, the estimation for the cost remains the same until you change the "Study Duration". In this sense, the balance of credits you see is not discounted, and you can make your prediction accurately.