how much money needed to trade S&P500 with local broker

Yes, you can buy 1 share (same as 1 unit).

But, $100 is not enough to buy 1 share of SPY (which is the index for SP500). LOL! 

What does "sales charge" mean?

Oh I see, you're referring to the transaction fee.

Are you paying a "flat" fee or by "percentage"?
If by "percentage", then it is time to ditch your local broker and open a direct one with US brokerage.

But, if you don't trade much (like once a month or a quarter) or a "buy-and-hold" type,
then don't worry too much about it.

Also, USD is high now.. perhaps everytime you buy/sell locally, they will charge you transaction fee, and currency exchange fee too.  That would be a double ouch.

min usd30.. what bank..? higher then than cimb itrade's usd25.
hope yr % is not higher than 0.4%.
anyway, if not trading much, minor effect...
i m on same thing: rm-sgd-usd. i find the rates reasonable.
what i do with say, sgd dividends, i leave it in trust acc for not more than a qtr, accumulate enough to buy more when price is acceptable.
come jan 2015, sgx going board lot 100 which will make things easier.
plan is to do same for usd, no reason to think rm will strengthen bigtime for "decades"; reverse may come true, though.
this oil story... after many saying it'll go as low as usd40 some weeks ago, now i start to read some reports making the case for usd64-65 avg for 2015. well, who knows.... but we do have an interesting opportunity now... to buy or to keep watching...?

$30/trade.. that's high.    Does this involve a human remisier assisted trade?

$30 could mean 30 trades for me..

yeah.. local brokers tend to charge round this range. but if you switch onto those US discount brokers like IB.. way cheaper.