If the co-sleeper is a portable, you may want to look into getting a crib that offers the option for one side to come off for the purpose of co-sleeping and purchase a good mattress. They do make really nice Serta and other name brand crib mattresses now. Many people will tell you it is not worth the money, but if you buy a convertible crib, it should also become a toddler bed. This means you could get up to 5 years out of the mattress...besides, her sleep and your sleep is SO worth it. Believe me, I have a 4 yo who still comes into our bed in the middle of the night and it started with co-sleeping in our bed to breastfeed and an uncomfortable crib mattress. Now that she has a twin, she stays in her own bed a little more, but we have developed a horrible habit with her!
Another trick, if your little one is waking up when you place her back in the co-sleeper, it could be bacuse the bed is cold. When you take her out to nurse, stick a large heating pad in the co-sleeper under the blanket. Before you lay her back down, pull the heating pad out and turn it off. Returning them to a warm toasty bed soemtimes makes all the difference!
Good luck from a fellow tired mom of 3!