Well Barry was a bit different because he was 33 and getting released. He also had a hell of a lot of miles on the clock. Delph was 29 - prime age for a midfielder really - and still under contract. Yes there can be value in the market, but it's not often a player in their prime who their current team don't want and haven't actually played in the position you have earmarked for him for about 3 years.I mean that's clearly not true as Gareth Barry was actually good and cost less. There is always value to be found.
I'm not saying never sign old players just not this particular one nor basically any other one who is being quoted at 30m minimum. I wouldn't want us to sign Luka Modric at that price.
Your logic can effectively be expanded to cover our entire squad. If Bolasie had been good he'd have been worth £25m, if Niasse was a footballer £13.5m would have been a snip. It just doesn't really make sense does it?