How much do you think it would cost to get a new battery for a iPhone X

$89 in done by Apple in the U.S. -- see: iPhone Battery Repair & Replacement - Apple Support

The price is approx $ 69.

If you want to know exactly, call Apple directly, see Contact Apple for support and service -> Contact Apple for support and service – Apple Support (UK) 

or 

call 1-800-MY-APPLE

See this Apple article: iPhone Battery Repair & Replacement - Apple Support

If you are going to replace the battery, see -> Get your iPhone, iPad, or iPod touch ready for service

Get your iPhone, iPad, or iPod touch ready for service - Apple Support