It really depends on what you're capturing. PATA? SATA? SCSI (I/II/III)? SCA? 1.8" IDE, 2.5" IDE? USB? Flash? SD? When you get down to it there's no cheap solution. You're likely to spend a lot just to cover the bases.
If you really need to budget then review what your most likely acquisitions are going to be. If you have a lot of legacy systems, it'll likely be IDE. Newer systems, SATA. High Availability servers? SCSI. Then price out one and figure something out for the others.
Some nice little devices that we use are the FireFly (SATA->Firewire) hardware write blocks. They're around US$200. You can find them here
http://www.digitalintelligence.com/forensicwriteblockers.php along with other forensic write blockers.
If you want to go the cheapest route, use a linux system with auto mounting disabled and buy some USB or Firewire drive enclosures. If you go this route make sure you create a documented procedure for acquiring evidence and follow it every time. You might even go as far as to record the history of your shell commands as part of your digital case file.
Tutorials : problem with use MSF
