How much would it cost to replace an iPhone 4s battery?

How much would it cost to replace an iPhone 4s battery? - 1

Added (1). Right now it's not charging past 61%, and it keeps jumping up and down from 61% to 62% (wouldn't charge past 41% the other day), all while it's plugged into a cord. Should I just save up for a new phone entirely, or replace the battery? (I've already calibrated the current battery and reset the phone.)

Just get a new phone. IPhone 4S is obsolete now anyway.

1st I would try a new battery charger - any USB charger… Any make… Or use your USB port on the PC…

if it still does this, then yes, it is likely that the battery is dead or charger circuit damaged.

Cost - will be the out of warrenty replacement charge (if you go to apple) - approx 150.

so given that it is a 4S. Gets none of the new features of the new OS,
probably best investment is a new phone (5c/5s etc), slim and fast…
rather than a 2nd hand/used iPhone 4S… Even a refurbished one from Apple.

but that is just my opinion.
and one on a 2 year contract, will be very cost effective (480/24 = 20 per month extra or less compared to a SIM only)

Your one-year warranty includes replacement coverage for a defective battery. You can extend your coverage to two years from the date of your iPhone purchase with the AppleCare Protection Plan for iPhone. During the plan's coverage period, Apple will replace the battery if it drops below 50% of its original capacity. If it is out of warranty, Apple offers a battery replacement for $79, plus $6.95 shipping, subject to local tax. Apple disposes of your battery in an environmentally friendly manner.

If your contract is up then I would definitely just upgrade because 5C is free on contract. You can then trade in the 4S also and get credit.