Is it expensive to study in Singapore for undergraduate and graduate degrees?

You can get a rough idea of how much it costs to get a degree in Singapore here:

https://blog.seedly.sg/uni-degree-returns-on-in...

It also depends largely on your lifestyle -- so if you're frugal and don't spend on big-ticket things or take taxis, then the bulk of your cost would really be just the tuition fees.
Keep in mind these fees vary for Singaporeans and international students, as with universities 'round the world.
And if you're looking to work in Singapore after your degree, this salary guide to median/maximum salaries across various job industries should help.

I think education is pretty affordable if you're a Singaporean. Maybe it might be a little pricey for foreigners.

30-50k for a 3-4 yr undergraduate degree is expected and I think universities in other developed countries would cost the same for their locals as well.