how much does it cost to fix the dents in ipad screen

Without benefit of an active AppleCare+ plan, a screen replacement will typically cost c.60% of the original cost of the iPad.

More details of iPad repair costs can be found here:

iPad Service & Repair - Apple Support

If you are referring to the aluminium alloy backcase that have edges that hold the iPad's scrern in place, that have these dents, the cost is expensive and prohibitive without a paid AppleCare+ plan.

If no paid AppleCare+ plan, just live with the dents.