What's a modern alternative idiom to 'Don't spoil the ship for a ha'porth of tar'?
Solution 1:
"Penny-wise and pound-foolish".
[https://idioms.thefreedictionary.com/penny-wise+and+pound-foolish]
"So concerned with saving money in any way possible that one fails to allocate money to things that will ultimately force one to spend more (due to lack of quality, proper maintenance, etc). I know you don't want to pay for this expensive course of treatment, but when ignoring your health lands you in the hospital and you have to miss work, you'll see that you were penny-wise and pound-foolish."
Solution 2:
buy cheap, buy twice TFD
poorly made or will wear out quickly
Or:
cut corners TFD
to take shortcuts; to save money or effort by finding cheaper or easier ways to do something.