1 Resource for, at the minimum, one card. Likely more however.
4 Resource for 3 damage and one card.
Effectively 0 resource for 3 cards.
3 Resource for, at the minimum, two cards.
2 Resource for minion invulnerability and one card.
2 Resource for 2 damage and one card.
3 Resource for 5 armor and one card.
3 Resource for, at the minimum, one card.
2 Resource for 1 damage and one card.
3 Resource for 1 damage across the board and one card.
7 Resource for 4 cards
And the list goes on..
Yes, you CAN use it more then once, but lets be realistic, due to it doing 2 damage to you, you probably aren't going to use it more then 8 times at the very most, and that's assuming you have some cards to heal yourself.
Not only is it too expensive, but why does it use mana..? Life tap restored mana. While I don't want it changed to 2 life for 2 mana considering that'd completely change Warlocks, it does need to be changed to cost no mana, or at the very least only 1 mana, and even that is still rather expensive.
Comparing it to other things that cost 2 mana and draw a card, life tap is by far the worst, and not even slightly worse considering you can use it more then twice, but it's just flat out horrible.
So, as stated above, it needs to be changed to either cost no mana, or less, or have less of a drawback or something..For two mana you can have multiple cards that give you both a benefit and card draw, making this hero power terrible, not to mention the fact that after spending mana to draw a card you might not have the mana to use the card you just payed 4 resource to draw.