Smalling being criminally underrated yet again in this thread
He was one of the first I thought of. We paid what, £12million? Had a long service out of him, he has generally been one of the top three picks for the England squad at CB until this year when Southgate wanted to try something different. We would surely be able to charge at least £20million in this market.
Phil Jones less so, but could probably ask for a similar amount of money (£20-25m due to fitness concerns). Might be wrong but if I remember right he cost us about £18m?
David de Gea is the most obvious. He is the best goalkeeper in the world and we'd more than double what we paid for him if we ended up selling. For some reason, 'keepers don't tend to go for as much as outfield players (likes of Andy Carroll and Lovren cost more than de Gea for example). However, this would be a case of bidding starts at £60m GPB or feck off, basically. As long as he was on a long contract.
Pogba is a tough one, but say Jose quit in the Summer and Paul didn't fancy playing for us and wanted that Madrid move a bit more, maybe we could get £90-100m, so slightly increased if his current form continues throughout the season.
Due to age, a lot of players wouldn't be worth more than what we paid (Valencia, Young, Fellaini, Mata, maybe Matic), but could probably fetch almost as much as what we paid still, due to how well they have been playing of late.
Disclaimer, these figures would be assuming we sell in the coming Summer window.