"One returned from the East Indies with but a moderate fortune of fifty or sixty thousand pounds, a diminutive nabob: a term borrowed from the chicken turtle."
-- 1811 Dictionary of the Vulgar Tongue
At this time, 'fifty or sixty thousand pounds' was actually quite a good amount; the key word here is fortune, rather than modest. "Chicken turtle" is a type of turtle that tastes like chicken -- a good replacement for chicken, but not the real deal. Thus, a 'chicken-nabob' is something like a 'mock duke'. These days we'd probably just say new money.