Ever hear of retrofits?
“A 737 winglet installation is estimated to cost $725,000 for parts and a 1-week installation process costing $25,000-$85,000.”
https://simpleflying.com/why-boeing-build-737-without-winglets/
Here’s an older story:
“Delta Air Lines has announced an agreement with Aviation Partners Boeing (APB) to purchase Split Scimitar Winglets for its B737-800 and B737-900ER aircraft.”
https://aerospaceglobalnews.com/news/delta-to-upgrade-its-737-fleet-with-new-winglets/