
Well, Singapore is opening up so it would be interesting to know the cost involved in travelling to Korea.
Flights
Step 1 travel lane to Korea starts from 15 November, by using Google flights the price is obvious.
It is roughly going to cost about:

This is just air tickets before taxes. So let’s assume about $1300 ( on the safe side), use a credit card so you get some miles or cashback for your travels.
PCR test
But even before flying off, you need to do a PCR test, whether you are a Singaporean or Korean it applies to all international travellers.
A quick google search shows that it is going to cost between $100 to $140 for one test.
Failure to submit negative COVID-19 (PCR) test results will result in denial of boarding at the departure airport.
Take note if your fling on the 15, you need to submit it within 72 hours before departure.
When returning to Singapore the test can be done for $160 including GST.
Hotels
Use the ShopBack addon on your browser to get cashback on booking.com, imagine you get 12% from ShopBack and another few more per cent from your credit card.

So we search for a cheap hotel for 7 nights and it is only SGD$350 and it includes taxes!!
Total
Flights: $1300
PCR before going: $140
Hotel: $350
PCR when coming back: $160
Cash: $1200?
Insurance: You need to get a quote for this
Total: SGD$1950 + $1200 + insurance = $3,150+++
In order to save more money try to use shop backs and credit cards to redeem some points.
There are a few credit cards that are best suited for travelling.