You have a couple different choices. The first thing you need to decide is if you want a 16 inch barrel (HK94) or a 9 inch barrel like the MP5 (Short Barreld Rifle).
The second thing you need to decide is clone or German.
The least expensive route is to go clone 16 inch. Even here, you have 2 options:
1) If you never intend to go short barrel, get a regular Bobcat BW5.
2) If you may eventually want to go SBR, then get a BW5FS, which has a short MP5 barrel with a barrel extension pinned on to get the barrel to 16 inches. When you decide to go the SBR route, and file the Form 1 with the ATF.... you can remove the barrel extension once you get the approved From 1 back from the ATF.
If you must go German (which many people recommend), you should look into the California Dept of Corrections HK94s that are on the market right now for around $2,500.
It costs about $1,000 plus or minus to convert a std HK94 into a SBR.