2 $ from one sale each time?

Hi everyone, I have a question, I get about $150 per month from sells.... but it's pennies... because the royalty is only $2, why is it so low? Sometimes it's $4, but I don't understand why there's such a difference. I understand that I can't compare this to RB, but with $2, there's not much motivation to develop the store. to get 1000$ i need 500 sells, this is crazy because 500 sales on the RB will give me 5000+... it seems to me that underestimating the prices of my designs does not seem fair, it dumps the prices of my art, people buy designs and not T-shirts...