I can't imagine anything that would take you to £3k's worth of upgrades. I had a "fully upgraded" SCAR NGR's with a Titan, Prometheus Gears, inner barrel....basically all that was left TM was the body, hop unit and trigger. That brought it up to a £1200 build (including cost of the RIF). I've just double-checked and Eagle 6 will sell you a brand new, maxed out 416 for £1450.
Anyway, I sold mine for £590. Bearing in mind I was the first owner and could list all the parts, plus demonstrate the work had been done by one of the leading NGRS techs. None of which applies in your case, so I certainly don't think you'd be "selling yourself short" if you were even in the same ballpark. Especially as people are staring to realise a "Fully Upgraded" NGRS does little more than a RIF costing a quarter of the price.
There's nothing stopping you listing it for something like £500 and then you can drop as your need to sell increases.