You don't need to spend a fortune on a knife but you don't want to buy junk either. For over 30 years now I've carried a Buck folding hunter. I keep it shaving sharp and I've dress many animals with it.
About 2 years ago I learned a trick for someone somewhere that for opening up a body cavity quickly and cleanly without fear of puncturing any internals the easiest thing to use is a standard utility knife with a carpet hook blade. (made for cutting carpets) I tried it and now swear by them. I use the Buck to do any other cutting and a small pocket saw for the pelvis.
https://www.google.com/aclk?sa=l&ai=...QwQvhd6BAgBEFo