I would suggest not basing your pricing on costs. Instead, base it on the value your users will perceive from your service. On reason is that the per-user costs are likely to be too small for you to measure. (Pricing based on costs also auto-commoditizes your offering.)

I second that actually, totally :)

I would just ensure that the freemium costs remain under control (that was the meaning I wanted to convey, at least).