From what I understand, if I want a transaction to finalised, I send to my peers I'm connected to, who send it to peers they're connected to, until eventually one of those people solve a block.

Whatever person solves the block which includes my transaction, gets, along with the 50 bitcoins for solving the block, any transaction fees offered in any transactions they've included in the block.

These transaction fees give nodes incentive to include transactions in blocks.

However, is there any incentive for nodes to propagate transactions. Say node A a transaction to node B. Is there any incentive for node B to send the transaction to node C, or are nodes just altruistic when it comes to propagating transactions?

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Also, I was just wondering, what's the current average transaction fees included in solved blocks? Is there a graph of this over time? I'm interested at the moment whether transaction fees or the 50 bitcoins for solving the block is greater.