I am searching from Amazon and found meters from about $20 to about $100. I don't know which one I should get. I just want a simple one for my hydroponic experiments.

Will the $20 meter do the job?

And I don't think I need a waterproof meter, do I? I am not leaving it outside in the rain or submerge it under water.

Here is a link that might be useful: pH Meters from Amazon