[Ethereum] Not enough funds for a transaction example in testrpc

I try to test one of my first Smart Contract in Solidity, a simple sub currency example, as you can see below :

pragma solidity ^0.4.0;

contract Coin {

  address public owner;
  mapping (address => uint) balances;



  function Coin() {
    owner = msg.sender;
    balances[tx.origin] = 1000;
  }


  function send(address receiver, uint amount) returns (bool success) {
    if (balances[msg.sender] < amount) {
      return  false;
    }
    balances[msg.sender] -= amount;
    balances[receiver] += amount;
    return true;

  }


  function getBalance (address user) constant returns (uint balance) {
    return balances[user];
  }
}

After migration and compile, please look my truffle command in the console

> var a = web3.eth.accounts[0]
> var b = web3.eth.accounts[1]
> var coin = Coin.deployed()
> coin.then(function(instance) {return instance.getBalance.call(a);})
{ [String: '1000'] s: 1, e: 3, c: [ 1000 ] }
> coin.then(function(instance) {return instance.getBalance.call(b);})
> { [String: '0'] s: 1, e: 0, c: [ 0 ] }

At this point I'm agree with these output, but when I want to make the transaction, they said I don't have enough funds …:

> coin.then(function(instance) {return instance.send(b,500);})
Error: Error: sender doesn't have enough funds to send tx. The upfront cost is: 724579920256558730963659928732440307373701337785 and the senders account only has: 99835768400000000000
    at runCall (/usr/local/lib/node_modules/ethereumjs-testrpc/node_modules/ethereumjs-vm/lib/runTx.js:97:10)
    at /usr/local/lib/node_modules/ethereumjs-testrpc/node_modules/ethereumjs-vm/node_modules/async/dist/async.js:3686:9
    at replenish (/usr/local/lib/node_modules/ethereumjs-testrpc/node_modules/ethereumjs-vm/node_modules/async/dist/async.js:882:17)
    at iterateeCallback (/usr/local/lib/node_modules/ethereumjs-testrpc/node_modules/ethereumjs-vm/node_modules/async/dist/async.js:867:17)
    at /usr/local/lib/node_modules/ethereumjs-testrpc/node_modules/ethereumjs-vm/node_modules/async/dist/async.js:840:16
    at /usr/local/lib/node_modules/ethereumjs-testrpc/node_modules/ethereumjs-vm/node_modules/async/dist/async.js:3691:13
    at apply (/usr/local/lib/node_modules/ethereumjs-testrpc/node_modules/ethereumjs-vm/node_modules/async/dist/async.js:21:25)
    at /usr/local/lib/node_modules/ethereumjs-testrpc/node_modules/ethereumjs-vm/node_modules/async/dist/async.js:56:12
    at Object.async.eachSeries (/usr/local/lib/node_modules/ethereumjs-testrpc/node_modules/async-eventemitter/node_modules/async/lib/async.js:130:20)
    at VM.AsyncEventEmitter.emit (/usr/local/lib/node_modules/ethereumjs-testrpc/node_modules/async-eventemitter/lib/AsyncEventEmitter.js:42:9)
    at Object.InvalidResponse (/usr/local/lib/node_modules/truffle/node_modules/truffle-contract/node_modules/web3/lib/web3/errors.js:35:16)
    at /usr/local/lib/node_modules/truffle/node_modules/truffle-contract/node_modules/web3/lib/web3/requestmanager.js:86:36
    at XMLHttpRequest.request.onreadystatechange (/usr/local/lib/node_modules/truffle/node_modules/web3/lib/web3/httpprovider.js:119:13)
    at XMLHttpRequestEventTarget.dispatchEvent (/usr/local/lib/node_modules/truffle/node_modules/xhr2/lib/xhr2.js:64:18)
    at XMLHttpRequest._setReadyState (/usr/local/lib/node_modules/truffle/node_modules/xhr2/lib/xhr2.js:354:12)
    at XMLHttpRequest._onHttpResponseEnd (/usr/local/lib/node_modules/truffle/node_modules/xhr2/lib/xhr2.js:509:12)
    at IncomingMessage.<anonymous> (/usr/local/lib/node_modules/truffle/node_modules/xhr2/lib/xhr2.js:469:24)
    at emitNone (events.js:91:20)
    at IncomingMessage.emit (events.js:185:7)
    at endReadableNT (_stream_readable.js:974:12)
    at _combinedTickCallback (internal/process/next_tick.js:74:11)
    at process._tickDomainCallback (internal/process/next_tick.js:122:9)

I tried with other accounts available in Testrpc, I've same results … May be I missed something in my command or set up I Don't know. But this cost seems to important, no ?

Thanks for your help !