depends on categories etc.. you are targeting some instances i have found adcenter cheaper others i have found adcenter WAY over priced..
All and all keywords are still cheaper (of course there are exceptions) in AdCenter but that gap is closing quickly as more advertisers are signing up and driving up the prices.
In my experience, adCenter is typically cheaper. I've also found this to be true of YPN, but I think both will begin to approach Google's prices as they continue to gain market share. Sam
msn adcenter has limited traffic compared to adwords. So, for scaling up, you may need to go with adwords. However, msn has its own market share, and needs to be considered by any ecom business.