Here are some tips on what to look for when weighing costs. Most service providers offer deals. Look into them. Some will give discounts or specials if your list is over a certain size and you move your list to their service. Or 1 month free if you sign up right now.
Deals are deals.
If you can save money without sacrificing quality, do so. Be wary of the extra costs a service might tack on and weight them into the equation. Are are they free up to a certain point, say 10 megs, and then they cost extra for extra space. Most service providers won't hide such costs, and will clearly state them in their pricing summaries. When weighing costs be aware of these things.