Hi Chatel,
Actually a better way to do it is to make a deposit, and subsequently exercise the option (must be within 14days) when the valuation is out. Nowadays, even for 4 rm flat there's no need to pay so much cash above valuation. What's more, this is a 5 rm whereby market is asking at or below valuation.
At the very least:
1. U know exactly how the flat is going to cost you.
2. If the valuation is high, U can neg to pay lesser on the cash portion. U have more control over things that way.
It's crappy for the agent to say that owner is asking for $30K. That is NOT your problem. If I'm the owner and I say I want to sell my place at $100K above valuation, does it mean that buyers must pay that kind of price? siao rite?
However, if you have already exercised the option, then you are binded legally. I can understand why u felt "cheated", but perhaps it's too strong a statement to say that agent cheated you since we are all mature adults. Perhaps you kind of regretted your actions of paying $18K cash above value for an unit? To be fair to all parties, I guess this is more like a case of willing sellers and willing buyers.
Cheer up ok!
U gonna have a place of ur own soon!