Penny stocks are the most expensive because they are the least reliable. Don't be fooled by a small per share price - that's not the measure of a stock.
Penny Stocks can be very rewarding if you've got 5-10 years real trading/investing experiance. Anything less & you're just gambling... with very poor odds.
On top of that.... you're asking strangers, whose qualifications and motives can never be known.... where to invest your money.