Currently have the i5-6500

Looking to spend 250 or less.

whats the best upgrade?

None for that budget, not worth it

What % of a performance upgrade would you say is worth it?

What do you do with your computer that it currently struggles with?

WoW is very processor heavy, not able to hit the frames i would like with a 1070 + current processor.

Have you verified with hwmonitor that it's your cpu holding you back and not gpu? What resolution/frames are you playing at? Are you streaming or multitasking while playing?

im at 100% cpu usage when frames are dropping. guaranteed its my cpu.

1080p resolution, trying to hit 144 frames. i bounce between 60-90 depending on the area im in. no streaming or multitasking

user bench mark shows this

UserBenchmarks: Game 53%, Desk 51%, Work 28%
CPU: Intel Core i5-6500 - 43%
GPU: Nvidia GTX 1070 - 77%
SSD: Crucial MX300 275GB - 66.1%
HDD: WD Blue 1TB (2012) - 87.7%
RAM: Unknown DDR4-2400 CL16 8GB 1x8GB - 24.6%
MBD: MSI B150M PRO-VD (MS-7996)

any suggestions?

Unless it's changed since last time I looked into it, WoW only uses 2 cores, so the game itself shouldn't really be pinning the CPU. I would check to make sure you don't have any background processes using the CPU too much.
Also, I've heard that not having an SSD can be a huge performance hit.
However if you want to upgrade, you'd be looking at something like this: https://pcpartpicker.com/list/fcTgjy (the h7 is $40 on newegg, it's just bugged on pcpartpicker).

According to that you're running single channel ram, that could be a problem. Also make sure you have wow installed on the SSD and not the HDD.