Cokin P vignettes unacceptably with the 82mm adaptor AND UWA adaptor on the 24-70ii
The OP stated just 24-70, but looking back the 24-70 II is what should have been stated. The bigger front element of the MkII is going to be an issue with the Cokin P, as it would with the 16-35L II.
Cokin Z will still exceed the OP's budget (holder, 77mm and 82mm adapters will be ~$150 with no filters, Cokin Z grad NDs are ~$70 each), but that's likely the best budget option.
Oh yea. My bad.
One more question. Are these filters truly neutral? No color cast at all?
5DMK3, M5, 24-70 2.8L II, 16-35 F4L IS, 70-200 2.8L IS II, 35F2 IS, 135L, 70-200L F4 IS, 600EX-RT x2, 430EX RT, EOS M IR converted, efm 11-22, efm 28 macro, efm 18-150, Fuji X-T20, Fuji 35 F2, Fuji 10-24 F4