That really depends on what the total cost to repair is. Apple charges $29 to replace a battery out of warranty. The rest of the cost really depends on what is actually wrong with the device. The good news about your 6s is replacing your battery should boost your performance quite a bit and it’s getting iOS 12 support which, believe it or not, should also boost performance. My wife recently had her 6s battery replaced and it runs iOS 11 like a dream.
Then again, if you can afford to flat out replace your 6s and were thinking about it anyway you could always trade/sell it and get an iPhone 8 or X. You should be able to fetch a decent trade in value even with a bad speaker. The Apple Store itself is usually pretty forgiving when it comes to buying back older devices with bad components.