It's difficult to say exactly and depends on so many variables including what mods you have already, what state the engine is in now etc etc but either way you will never really get a lot extra out of these cars without going FI or by having a built engine.
It's not all about the extra Bhp you gain though as it can help make the car's drive ability much better especially if you have added mods as the UpRev mapper will be able to adjust everything to run smoothly together giving you hopefully a more linear power curve. Have a read of the info below from Horsham Developments where I had my two UpRevs from (once on my Zed when it was N/a & again after I went FI).
Source/link to Horsham Developments: http://www.h-dev.co.uk/index2.php?page=350z_mapping
Price wise Horsham are quoting £399.99 for the UpRev on an N/a Zed.
As an example though when I had my car (2003 350Z DE) remapped the first time it went from 282.7Bhp to 293.4Bhp and it had these mods: Berks HFC's, Invidia Gemini Exhaust system, Horsham Developments plenum spacer & a Stillen air induction kit.
So on paper it only gained around 10Bhp odd but with the remap the 155mph top speed limiter was removed, the initial acceleration in the first few gears was greatly improved and I felt it made a good improvement to the way the car drove overall.
Whatever you decide to do though it is always best to get all your performance related mods done first before any UpRev as otherwise you'll just have to pay to have another UpRev done again if you add mods at a later date.
Fyi ~ Regarding the EML being on you can buy code readers from eBay for around £20-£30 which can read the codes for you so that way you'll know for certain what it is and also you'll be able to turn the EML off, ....although it will come on again if it's the De-Cats/O2 sensors causing the EML in the first place.