In the last 30 years:- Walker, Lowton, Quinn, Sharp, Jagielka, Britton, Kenny, Kelly, Derry, Dellas, Mendonca, Beagrie, Marsden, Cockerill, Edwards
I did say the last 10-15 years
I'll give you Kelly and we didn't have much of a choice with Jags or Kenny(or they wouldn't have signed the contracts) but the others weren't undersold were they...all of them we got what most thought was the right price:
Lowton- not good enough at premier and possibly championship level
Quinn- we had to get rid of the wage. We didn't have much choice and many thoght he was awful
Britton- no one knows what we got for him but we got him for nothing
Derry- £750k for a utilty player was more than we thought we would get. Warnock had to rebuild and need the cash
Marsden- journeyman footballer who we got cash for
Dellas- he decided he wanted to go back to Greece- we didn't have a choice
Mendonca- tried him twice,didn't work- only really performed at Charlton below the top tier
Beagrie- £250k plus a sell on if I recall for a was an excellant price in the era we sold him
Cockerill- again a big fee in the era we sold him
Edwards- no one ever paid big money for him- strange but maybe managers saw him as too much of a luxury player and wouldn't pay.
In contrast I give you Wayne Quinn,Curtis Woodhouse,Jacob Mellis,Jordan Slew,Paul Williams,Michael Tonge(i sure I will think of others given time) where you could argue we robbed the clubs who bought them