Just reflecting out loud here, as I struggle with this as well.

The way you said it makes me think that at $4/month, you might actually be competing with free, as the prices seem so close. If it's $20/month, they may not say "oh but why isn't it free" or even more so at $50/month.

At low prices, maybe it just seems too much of a hassle to sign up and pay, ie, "Ugh, just $4/month? And I have to go thru the burden of signing up and giving my credit card and all that?" Whereas at $50/month, the paying process pain may feel relatively smaller.