If you purchase 5 or more domains, you can get free whois privacy, basically, for every other domain that you've got in your cart. So to make it as cheap as possible, I'll usually take 5 .info domains, and apply that benefit to any other domain(s) that I have without it (whois privacy).
If you've got a bunch of domains in your account that you'd like to apply privacy to, check all of them, and go to Upgrade in the domain panel. That'll take you to your cart. Then, you go and bulk register 5 .info domains (I usually try to think up .info's that'll actually be useful) and apply privacy to them on the way to checkout. When you're back in your cart, you should see that all the privacy prices are down to $0.00, leaving you to pay just the prices of the .info domains. If there are any .com domains that you'd like to purchase right then and there, you can buy them and get the benefit of free privacy too (as long as you apply it on the way to your cart).