Check your settings again for EVO parameter.
In OMFI, you have to set the value as do not copy.
The same value would be in user parameter.
Then PO price will be always copied from info record condition (not from last purchase order) or if you are creating reference to contract, then price will be picked from contract.
I've tested that, it is working fine.
If you have still problem, then please share screen shot for settings.