[Ethereum] trouble matching gas used in transaction with difference in account balance

I've fired up my testrpc and deployed a simple HelloWorld contract using truffle. I send a simple transaction to my contract and I log my account balance before and after the transaction. Unfortunately, this does not add up to the gasUsed parameter I see in the transaction.

contract HelloWorld {

    uint public x;   

    function HelloWorld() {
        x = 5;
    }

    function set_x(uint _x) returns(uint x) {
        x = _x;
        return x;
    }

}

And my javascript

var HelloWorld = artifacts.require("./HelloWorld.sol");

module.exports = function(callback) {}

from_wei = web3._extend.utils.fromWei

var account1 = web3.eth.accounts[0]; 
var start_balance = web3.eth.getBalance(account1).toNumber();

HelloWorld.deployed().then(function(instance) {

    return instance.set_x(500)

}).then(function(tx) {

    console.log(tx)
    var new_balance = web3.eth.getBalance(account1).toNumber();

    console.log(start_balance + " initial balance");
    console.log(new_balance + " balance after transaction");
    console.log((start_balance - new_balance) + " difference");
    console.log(from_wei(start_balance - new_balance) + " difference (from_wei) ?");
    console.log(web3.eth.gasPrice.toNumber() + " gas price")

})

Outputs something like this:

{ tx: '0x1a2876b330618c75c7e264becbee75c491b47b5a17a0f9f9fe7fb124f0e93113',
  receipt:
   { transactionHash: '0x1a2876b330618c75c7e264becbee75c491b47b5a17a0f9f9fe7fb124f0e93113',
     transactionIndex: 0,
     blockHash: '0xf30134cac3be167d6053a21953b87010ce7145ccb36775e76d38458dddfa81b9',
     blockNumber: 16,
     gasUsed: 21736,
     cumulativeGasUsed: 21736,
     contractAddress: null,
     logs: [] },
  logs: [] }
99939363900000000000 initial balance
99937190300000000000 balance after transaction
2173600000000000 difference
0.0021736 difference (from_wei) ?
20000000000 gas price

It looks like the difference in my account balance IS the same as the gas used, just a different magnitude. So I'm confused? Are these numbers expressed in wei? I thought I needed to multiply by the gas price to see how much the transaction would affect my account balance…? How can I get GasUsed to equal the difference in my account balance?