Why does it need to be a huge sum? Even if you don't like Henderson, why does a replacement goalkeeper for De Gea need to be more than the £25m or so we'd probably get selling him?
You look back at last summer at goalkeepers that would be substantial upgrades moving for fees considerably lower than £25m, Maignan moving clubs for less than £15m (I'm still not over how we just sat by and let that happen tbh), Jose Sa for less than £10m, Donnarumma on a free, Kobel less than £15m. So yeah, the goalkeepers are there, you just have to look for them.
I can only presume you've never watched a 1980s lower league goalkeeper if you think De Gea is a throwback to one.