What constitutes affordable? For pairing with a 57, don't bother with a 7b. The 7b is nice, but it largely does a lot of the same things that the 57 does. For blending with a 57 an MD421 would be abetter choice IMO, if you want clarity. For warmth a Cascade Fathead would be a great option too, but read up on care and handling of ribbon mics if you choose to go the fathead route.