Phono preamp upgrade price/performance

Im wondering how much I would have to spend on a phono stage that would be better than the built in one on my amp?

amp - Arcam Radia A15

TT - project debut carbon with 2m blue 

so i think it sounds pretty good but I recently tested a different amp (Marantz model 50) and I thought ithe phono sounded better than the Arcam. I didn’t keep the Marantz for a bunch of other reasons, but it got me wondering, how much better can things be with a “better” phono stage and what would it cost.