Tegan from looking around, I would say between $20 & $25...
The baby beehinds are $25 & then the cover PUL was $10...
But they can go from newborn to toddler!!!
which to me is a huge effort!
Also some All In Ones are great coz they are funky on the outside & so not cevered by a plain cover! These sell for about $20ish, I have grabbed most from e-bay (I buy new)...

HTH....
also check out Fionajills post below!