Tanglin Trust is in District 5 but not all of district 5 is expensive, especially if you stay a little further out. Districts 10 and 21 are still within reasonable reach of Tanglin and are a notch cheaper than district 5.
I work very close to Tanglin and live on the very edge of district 5 but have lots of colleagues who live further out in Districts 10 and 21.
There is next to no accomodation within easy walking distance of Tanglin so you or your kids will likely go to Tanglin by bus or taxi or.... so you don't need to live in district 5 if the rents seem high to you.
By the way, this website tends to show the more expensive condos
. Quite a few cheaper, older and still very nice ones (the old ones are often bigger units)are not listed on this site.