under $100 gets you quite a range in fact whether new or used. guess it also gets depending on the 'style' of your bike? frameset color, components and most of all what 'saddle' on it etc. any 'particular' theme you'd like to achieve? (out of curiosity).
one of all mentioned above, tho, i've found that all the vintage SR seatposts (not Campy SR but Sakae Ringyo SR) are exceptionally heavy. no-idea-why-an-alloy-post-should-be-that-heavy sort heavy.
IMO, nothing beats Campy C-Record Aero, especially a polished one. yes, i've seen ones under $100. bit a of time & luck, you'd need, tho.
good luck!