Depends on what you're trying to achieve in the league. With only 20m, might be better just to sign a great CB and then go with some cheap veterans at LB/RB who won't make too many stupid mistakes and have them play a bit more conservative.
If you're looking for cheap, I think at some point on every FM game I've played in 07 and 08 I've bought David Mendes da Silva. He's pretty reliable for a middle of the road team. Definitely not as spectacular as the expensive players
ppl like throwing around on these forums.
LB, I bought Jeremy Mathieu for Reading after selling Shorey. After a few games, seems to be steady.
RB, don't know. Eduardo Ratinho looks pretty cheap in this version with some still decent stats.