I'm doing all dry rock on my next build (many reasons for this, with cost being one, although far from the most important reason for me). Some people do a mix of mostly dry with a little seed live rock. Others have some dry base rock and all the visible rock being live. Just different options to save some money.
As for live rock, there are different types. There is the random type just sitting in a bucket in the LFS with bacteria on it but not much else visible. There at least was (not sure if it is still available from anywhere) what is called boat rock since it was shipped from overseas on a boat and had a lot of die off but still a lot alive and was a cheaper live rock option. Then there are things like the Florida live rock that can be shipped in bags of water. Those are full of life and obviously cost a lot more. The vast majority you will find in a LFS should be on the cheaper end of $7-8 as LiveWire said.