Try the Eunos Industrail Park. Just next to the Singapore Post Centre.
Depending on the vespa you want.
I got one at $400/- Sold it off at $800 after 6 months of riding it.
So you surely wouldnt want to buy from me
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It will depend on its condition lah.
Those pretty done up antique vespa of course cost more. Usually starting from $1000.