note: The products linked to here are just for example sake and to cite example prices. I haven't used these products and I can't say if they're good so I'm not specifically recommending them. Its smart to do your own research on such things.
Greeting Card software & Printing
Cost : Software = $13+ , Paper & Envelopes = 50¢ ea.
You can get computer software like Hallmark Card Studio Deluxe 2010
for $13.28 and then print cards at home. The software has thousands of designs and lets you do lots of customization. The reviews on the software available is mixed and some people have problems getting it setup and working well. Special paper to print the cards will make them nicer quality than simply printing and folding plain paper. A 20 pack of Avery Half-Fold Greeting Cards for Inkjet Printers
will cost you $7.22 or you can get 30 Textured Half-Fold Greeting Cards for Inkjet Printers
for $11.18. You'd have to get envelopes separate. A box of 100 Invitation Envelopes 5-3/4" x 8-3/4" is $11.54 You can also get 30 Avery Half-Fold Greeting Cards for Inkjet Printers (8316) with envelopes
for $15.21Online Services
Cost : Subscription for $15-20/yr, Cards & Envelopes = 50¢ ea.
There are several online services that let you create and send greeting cards. Hallmark, American Greetings and others are available. Top Ten reviews site gave American Greetings the #1 ranking in their reviews.
American Greetings has subscription plans for $3.99 per month, $19.99 for a year or $29.99 for two years. With that you can send unlimited e-cards and you also get unlimited print at home cards. The service is pretty easy and gives you tons of options and flexibility.
General Purpose cards
Cost : 40¢ to $1.10 each
A few weeks ago we bought some general purpose cards at Costco. I don't remember the exact price, but I think we spent around $20 for 20-30 cards. These cards look great and they are really easy. I found a similar product at Amazon this pack of 30 cards is 
$32.95 : 30 Handmade All Occasion Greeting Cards with Decorative Reusable Organizer Box
These are pretty fancy cards too. There is also a cheaper and plainer All Occasion Card Asst, Set of 30
for $12.95. My mom used to buy cards from a company named Current. They sell a set of 20 general occasion cards for $5 + $5 shipping. If you buy a couple packs you could get 50 cards for under $20.Hand Made Cards
Cost : Free to 50¢ each
You can always just grab a piece of paper and write your own message on it. This can be essentially free if you ignore the marginal cost of a piece of paper. You can also buy printable blank cards and envelopes like those mentioned above and use those. Making cards by hand that look decent requires some sort of artistic skill so this option isn't for everyone. It can also be more time consuming. However a nicely hand made card can add a personal touch and the extra effort may be appreciated.
Thrift Stores
Costs : variable
More than once I've seen unused greeting cards for sale in thrift stores. You may be able to pick up some nice cards for a bargain price. However the selection will be extremely random if you can find cards at all.
Bottom Line: There are a variety of ways to get greeting cards for closer to 50¢ to $1 range and by doing so you can save yourself a fair amount of money over a year.
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