depending on how well u know the fellow and how polite you wanted to reply him.
But this never fail and will normally hit him on his face.
"because i like the watch and i can afford it"
your friend will reply, why don't u invest in blah blah blah and i'll reply "i already have some of those, and I really like the watch, so I don't mind having it"
and if he still insisted the RM20 watch will work just fine then i'll reply "women have their branded hand bags e.g. LV, Gucci, Chanel. Men have their watches. The free Isetan plastic bag works fine, why did you get her branded hand bags?"
if he didn't get any of those, he just being envy of u.
if he's rich and doesn't get it, he's either stingy or being humble. If he's being humble, he would have understand that his RM20 watch is a junk. Else he's just being ignorant.