i think the additional amount is taking into consideration of
1) The surrounding- walking distance to amenities,
Train station, Malls, bus interchange etc. Bidding price of
such land confirm much higher than other area.
2) Some dbss like AMK/bishan/Toa Payoh ( i cant remember
which one) they actually put a gate and requires access pass upon
entering. Each household will have a biometic door. that is
somewhat similar to condo
3) I can understand your point of view for the 100k
renovation on HDB BTO. but bear in mind you need to fork out
cash for the 100k renovation. there is no installment payment
for that 100k unlike DBSS - i doubt any contractor will accept
installment basis. If you are judging from time value money, that
100k is better off use as investment for shares than
renovation.
For the 100K renovation, you can get a bank loan.
But I did not know that DBSS is exclusive to the public ie.
cordoned off with a gate. Now that you mentioned it, I recall
the Pinnacle at Duxton has this concept.
I am presuming the normal HDB rules apply - 5 year MOP, HDB
standard rules in renting out of flats, rules regarding potted
plants etc. 
