You could try Gumtree as well for second hand good bikes in which imo you need to know about Mountain bikes before going to look at them so you would know what kind of deal you are getting.

Most average bikes cost about $400+ for me to meet the "good" enough mark.
This is from my point of view, not a bible.

I know bikes range from $200 -$5K so it just shows how much you will spend and go.

Important factors are getting the right size frame for your body, right frame material that you like, and weight that you are comfortable with.
Followed by "do you need full suspension front and rear?" or just a hard tail?
What speed? (gear ratio)
What kind of gear shifting? Grip or rapid fire
Brakes, disc or normal wired?