I am quite new to formula. How can we write formula for - I want a checkbox to be set as True if Price value(Number) is more than 30.
As mentioned in the comments, you'll definitely want to use Trailhead to get a general overview of formulas. In this case, it's the IF function that will do the trick:
IF(PriceField > 30, true, false)
Edit: as pointed out in the comments, the IF/True/False part isn't strictly necessary. You may omit them if you become comfortable with formulas and don't have junior team members who need things explicitly spelled out.