Not sure if I'm misunderstanding the post, but the game already works this way. If you die more, you're worth less unless you reset your gold value with a kill. Assists don't reset gold values.
League of Legends Kill Values (wiki):
When a champion is killed, the player who scored the final hit receives gold and experience. Additionally, a champion with many consecutive deaths will award a significantly reduced bounty, while one with many consecutive kills will award a higher bounty (refer to Bounty values Notes below for details).
Edit: Unless you mean that player (the Tank) is worth a standard amount of gold regardless of deaths and their allies become more valuable to encourage varying kills?