I think that Lloris is comfortably better than Navas. If we really sell De Gea, we should insist on a deal with Madrid which says that we sell De Gea for that amount of money we can get Lloris (similar to Torres-Carroll transfer). Failing that, keep De Gea for an another season and then go all in for Lloris. Or just go now for him 30-40m and sell De Gea for whatever Real pays (20m+).
I think that a great keeper, who won't have problems to adapt to the league is very important. Cech could be another good solution, but I don't think that he has been that great since his injury (though is very reliable).
How does that work? Real can have De Gea for free next year, they're not going to agree on that and besides I don't think anything of a sort has ever happened.
We'd have to pay at least £50m for Lloris next year, possibly around £70m. It'd be the worst deal ever. There's absolutely no way Levy would sell him to a Premier League side for £30m or even £40m. I guess it'd be around £60m realistically.