There is no bottom end x99 cpus.
A quick search shows that a 3970x goes for ~$300 normaly. And the cheapest one currently listed on fleebay is $375 for a used CPU.
A 5820k is faster in every way and is currently $370 for a new one from amazon.
x79 motherboards will be more expensive, harder to find, and have no warranty.
I went through this same decision last year. Older cpus/mobo have held there value more than normal since 2nd gen icore.