I used a light weight one from babycity, they have polarfleece ones up to $40, and the one I got was on special from $29 down to $5. We use ours all the time, not so much now Haleighs older, but we did and will def with this baby.
Baby factory sell the expensive ones ranging from around $50 up to over $150 (well they did in lower hutt when i last looked).
I highly recomend them if you have a baby who has the habit of kicking off blankets, or your baby is born over winter, they can also stop baby from slipping under blankets in the night as they are already ina blanket.
We have 2 but we needed a few more as one was for pram and the other was for bed and we had to wash it often, especially with little sccidents like sick baby

. So prob 2 - 4.