I just laugh it off. They are obviously ignorant.
Had one once, a shop we clean I walked in to collect payment, a 'lady' customer made a comment along the lines of 'where's your rags and newspaper ha ha ha'
I replied 'I left the in my holiday home, ha ha ha'
Smiled smugly, then hopped back into the RS6 - the look on her face (I'm not rich by the way, but do ok).