Ki Sung-Yueng might be worth a punt, seems to have done well at Sunderland and not sure why Swansea lent him out as done well there last season.
Caulker as mentioned would be good. Ruddy not sure much of an improvement, if at all over Boruc.
Giaccherini from Sunderland, Yacob, Berahino & Redmond from WBA, Hernández, Michu, Bony & Vorm from Swansea would be others who would be worth a go at the right price, from the other teams that could go down.