wow nosy aren't we? j/k
True but how are you supposed to help someone without knowing all the nitty gritty?
You can buy them for $5, $10, $15, $20 which give you varying amounts depending on the card. For instance I've seen on a $20 anywhere from 100 minutes to 528 minutes to Spain. Connection fees also vary from 0 to $1.50 for each call placed. I've looked at dialpad and net-to-phone since posting and they seem to be more expensive than just getting some of the cheaper cards. I found one place that will sell a $20 card for $19.50 and it gives you 528 minutes to Spain. The connection fee is $.99. I just ordered it and should be getting it within a half-hour by email. 
First of all I have yet to find a PC to phone program which offers as clear call quality and the convenience of phone-to-phone as these pre-paid calling cards you find littering gas stations and mini-marts.  For the past few years I have been nothing but pleased with (get ready I'm about to forever change your life) 
BigZoo.com.
I like them because:
1) There is no card it's just an 888 access number and your PIN (which never changes).
2) Your PIN is super easy to remember it's your home phone xxx-xxx-xxxx plus four digits they assign you.
3) NO CONNECTION FEES! (unless you're calling from a payphone - then it's 55 cents I think)
4) The rates are very low and competitive for domestic and international long distance.
5) Call quality to Spain is decent at least to Madrid and Barcelona which are the only two cities in Spain I have called.
6) You can recharge your PIN online instantly at bigzoo.com in either $10, $20 or $50 increments and assuming your credit card is legit you'll be credited the extra minutes instantly.
7) I'd suggest starting out with $10 to see how you like it.
8) I love their online billing statement it shows details of all my outbound calls with all sorts of stats for the past four months including the present month.
When you consider there is NO CONNECTION FEE then they are comparable, your store bought pre-paid cards may offer more minutes but in the long run I'd have to say that BigZoo.com offers a more compelling package and overall value.
Here's a good 
long distance thread found in the Ferengi forum.
WTF am I doing in banking? I should be a car salesman with slick hair and a fake Rolex!
