The thing that will work in our favour (and something that shocked me this morning) is that Sterling is still only 26.
His value has weakened because of recent (2+ years IMO) form and contract situation but we'd be right to figure his value out based on what we got for Sane.
No idea what FDJ is valued at but they got him for £65m in 2019 when his stock was high.
The £75m figure from the media today feels plucked out of nowhere because, like you said, the Sterling valuation will only be known by us/Barca. Both clubs can exaggerate whichever fee they want to fit the narrative if it does happen, though.
FDJ has a release clause, of €400m. So his value really is either that, or whatever Barcelona accept for him. They are unlikely to accept less than they paid for him imo, but would not pick an amount that stops the deal happening either. Therefore 75m is not a daft guess really.
We I would imagine we value Sterling at whatever money we are prepared to pay as an extra, minus whatever they value FDJ. So, if the club are prepared to pay up to 20m including Raz, and they value DJ at £90m, then we value Sterling as £70m. If they value DJ as 50m, we value Sterling 30m. etc.
If both are happy to do a deal, the final value wil mean whatever each party wants it to mean. The only real and meaningful value will be that extra that they get paid for Raz, the rest is for reporting purposes only.
The fact we are supposedly willing to loan him initially must also factor into the cost of the extra, to jeep it down, you would think.