For clothes it would be a navy blue, pickstitched, RLBL sportcoat for 100 bucks
A 3 piece tweed suit by RLPL for 400 (best suit i own so far)
For other things however...
I collect and sell comics and antique violins.
Usually I find violins that are worth about 500-1000 for about 150-300 dollars in price at estate/garage sales/charity auctions. My best find, however, was a pristine condition Pietro Varini Violin, dated circa early 20th century, which ended up going to a member of the Los Angeles Philharmonic for 5400 dollars. I paid 950 dollars for it as the winning bid at an estate sale.
Comic books have much smaller margins, but have relatively low purchase prices. I found an issue of Spiderman number 300, 122 and 238 at a garage sale (each worth from 150-350 dollars depending on timing) and I bought them for a dollar each. Their son, an avid collector, had died in an accident and they needed to get rid of anything that reminded them of him. Those comics found a good home though.