You lose life equal to that spells converted mana cost
So far, Oblivion Sower is the only one that can provide mega-value, since it can get back lots of lands at once
The low mana cost of Shadowspear is what makes this card such a good answer: the equipment itself is just 1cmc and the ability -- which doesn't even need you to equip first to use -- is also 1 mana, making it super easy to tutor up and use it when combined with follow-up removal
With all that in mind, here are some cards Id recommend checking out
Interestingly, while it sounds so much better on paper, Cub may not actually replace Bristly Bill in every format
Much like Gifts Given, its a card that should be taken early and if so can be built around by taking incidental mana fixers to support it, like a Golgari Rot Farm in a B/U control deck