How much would it cost to fix dent on backside of iPad?

Perhaps due to regional and model differences, labor could vary

and exceed the value of several smaller iPads. ~ Even if so, there

might be a better bet, with AppleCare+ as the product may meet

an exchange program for less than retail cost.

Genius Bar Reservation and Apple Support Options - Apple

//www.apple.com/retail/geniusbar/

In any event, you'd still best consult an Apple Store genius or

'Apple Authorized Service Provider' for their opinions, on that.

For practical purposes, in lieu of repairing and returning your own iPad, Apple usually exchange the iPad for a renewed/refurbished iPad of the same model and specification. The cost of the exchange is the same as would otherwise be charged for repair. Unless you have benefit of an active AppleCare plan, an out-of-warranty repair while expensive will be somewhat less than the cost of a replacement iPad.

As for out-of-warranty repair costs, Apple offers a fixed-price price repair. You can verify eligibility and cost for your region/country here:

iPad Repair & Service - Apple Support

If you have an active AppleCare+ plan, the cost of repair is reduced to a small excess/deductible fee.

Which iPad (model and generation)?

iPad Repair & Service - Apple Support