[Devolver]

You're competing with SumoMe, which does essentially the same thing + is free (with one-off cash charges for upgrades/prettier templates, etc).

What's the differentiation here?

[gt5050]

SumoMe is not exactly free(they have paid upgrades as you suggested). The differentiating factor is the WYSIWYG pop builder which lets you completely customize the popup to match your website.

[AznHisoka]

So your differentiating factor is an UI feature that can be duplicated in a week?

[Devolver]

$19-499/month seems pretty expensive for that.