You will spend around $6500 for a nice M16, around $4500 for an SP1 conversion. If you want to shoot .223 but spend less, you can buy a Ruger 556 for around $3,000. If you get in on a volume buy, you can get a new Vector Uzi for about $2800, a real bargain.
A decent MK760 will run you just over $2k, a Stemple about $500 less. But for maximum bang for the buck, you can't beat a MAC. Depending on condition and type, you'll spend $1,000 to $1,500 and have a great time. A Powder Springs MAC10 in 9mm or .45 is a very fun gun. Add transfer taxes to all these prices.